If a lord was proceeded against then the HoL would be the venue of first instance. But where would the appeal go from there, and what was the venue of last resort? Or was it there be all and end all, ie at one and the same time the court of both first instance and last resort
There were 30 House of Lords convictions of peers resulting in a punishment for the peer (in 29 cases a death sentence, and in the one final case, in 3 months of imprisonment) from 1499 until the practice was abolished in 1948, a time period covering all but the first 158 years of the 607 years during which House of Lords trials were available and the process was formalized.
Of those convicted peers, 30% were pardoned or commuted by the monarch. There was no appeal from a House of Lords conviction other than a royal pardon or commutation.
Modern criminal appeals did not exist in ordinary criminal cases either, in any part of the time period during which peers were convicted by the House of Lords of crimes for which we have good records (i.e. from 1499 to 1901).
Ordinary criminal procedure compared
This should also be viewed in the context of the history of ordinary criminal procedure in England (this link is the source applies to all of the discussion in this section).
Prior to the year 1700, appeals consisted of a petition to the King or Queen resulting not in a pardon or commutation, but in a direction by the monarch for officials to determine if there were any serious errors in the process by issuing a discretionary writ of error, so it basically, another form of pardon type relief.
The Writ of Error and some other procedural remedies short of an appeal existed from 1700 to 1907 in England but evolved over time.
Initially, review other than discretionary review by the monarch was limited to a determination that the judge was really a judge in good standing with jurisdiction over the case, that the indictment described a crime, that a correct number of eligible jurors were seated and rendered a verdict, that the sentence was consistent with the sentence authorized by law. This kind of review of criminal convictions was similar to 18th and early 19th century habeas corpus review of convictions in the United States. Writs of error were also used discretionarily in the U.K. in this era in cases where the prosecution wanted to admit that it made a mistake and vacate a conviction.
A detailed record of the proceeding that would make a more substantive review possible, however, was not available until 1886, and that was replaced, 21 years later, by the modern direct appeal of a criminal conviction based upon a trial court record include a transcript of the proceedings and all exhibits that had been admitted in the case.
True formal direct appeals of criminal convictions in the modern sense, however, did not exist in England prior to 1907, after the last verdict of conviction was made by the House of Lords. So, there weren't true direct appeals in ordinary criminal cases either, in the time period when it would have been potentially relevant to appeals of House of Lords convictions.
Of course, the monarch had the power to pardon or commute the sentences of people convicted of crimes in the U.K. in ordinary criminal cases, "since times immemorial", just as in House of Lords cases. But, people with ordinary criminal convictions, unlike peers convicted in the House of Lords, did not have the right to demand an audience with the monarch in the way that a peer did, which made the process of seeking pardons and commutations much less effective for commoners.
The right to be tried by other peers was established around the time of the Magna Carta, formalized in 1341, and formally abolished in 1948. Prior to 1695 it was before only a jury of nobles hand picked by the monarch to rule as the monarch desired, when parliament was not in session, but after that the entire House of Lords participated in each trial.
From 1547 to 1841, there was no punishment after a first offense other than treason or murder, which was just a strike to open up the possibility of punishment for a second offense, and greatly reduced the need for an appeal. According to Wikipedia (references omitted):
The right of peers to trial by their own order was formalized during the 14th century. A statute passed in 1341 provided:
Whereas before this time the peers of the land have been arrested and imprisoned, and their temporalities, lands, and tenements, goods and cattels, asseized in the King's hands, and some put to death without judgment of their peers: It is accorded and assented, that no peer of the land ... shall be brought in judgment to lose his temporalities, lands, tenements, goods and cattels, nor to be arrested, imprisoned, outlawed, exiled, nor forejudged, nor put to answer, nor be judged, but by award of the said peers in Parliament.
The privilege of trial by peers was still ill-defined, and the statute did not cover peeresses. In 1442, after an ecclesiastical court (which included King Henry VI of England, Henry Beaufort and John Kemp) found Eleanor, Duchess of Gloucester, guilty of witchcraft and banished her to the Isle of Man, a statute was enacted granting peeresses the right of trial by peers.
By the reign of Henry VII of England [reigned 1485-1509], there were two methods of trial by peers of the realm: trial in the House of Lords (or, in proper terms, by the High Court of Parliament) and trial in the Court of the Lord High Steward. The House of Lords tried the case if Parliament was in session; otherwise, trial was by the Lord High Steward's Court.
In the Lord High Steward's Court, a group of Lords Triers, sitting under the chairmanship of the Lord High Steward, acted as judge and jury. By custom, the number of Triers was not fewer than 23, so that a majority was a minimum of 12, but in fact, the number ranged from 20 to 35. The power to choose which peers served as Triers lay with the Crown and was sometimes subject to abuse, as only those peers who agreed with the monarch's position would be summoned to the Court of the Lord High Steward, thereby favoring the desired verdict.
This practice was ended by the Treason Act 1695, passed during the reign of King William III. The Act required that all peers be summoned as Triers. All subsequent trials were held before the full House of Lords.
In the House of Lords, the Lord High Steward was the President or Chairman of the Court, and the entire House determined both questions of fact and questions of law as well as the verdict. By convention, Bishops and Archbishops did not vote on the verdict, though they were expected to attend during the course of the trial. They sat until the conclusion of the deliberations, and withdrew from the chamber just prior to the final vote. At the end of the trial, peers voted on the question before them by standing and declaring their verdict by saying "guilty, upon my honour" or "not guilty, upon my honour", starting with the most junior baron and proceeding in order of precedence ending with the Lord High Steward. For a guilty verdict, a majority of twelve was necessary. The entire House also determined the punishment to be imposed, which had to accord with the law. For capital crimes the punishment was death; the last peer to be executed was Laurence Shirley, 4th Earl Ferrers, who was hanged for murder in 1760.
From 1547, if a peer or peeress was convicted of a crime, except treason or murder, he or she could claim "privilege of peerage" to escape punishment if it was his or her first offence. In all, the privilege was exercised five times, until it was formally abolished in 1841 when James Brudenell, 7th Earl of Cardigan, announced he would claim the privilege and avoid punishment if he was convicted of duelling. He was acquitted before the introduction of the bill.
The last trial in the House of Lords was that of Edward Russell, 26th Baron de Clifford, in 1935 for manslaughter (he was acquitted); the following year the Lords passed a bill to abolish trial by peers but the Commons ignored it. The right to trial by peers was abolished when the Lords added an amendment to the Criminal Justice Act 1948, which the Commons accepted. Now peers are tried by juries composed of commoners, though peers were themselves excused from jury service until the House of Lords Act 1999 restricted this privilege to members of the House of Lords. The right to be excused was abolished on 5 April 2004 by the Criminal Justice Act 2003.
Peers were and still are, hypothetically, subject to impeachment. Impeachment was a procedure distinct from the aforementioned procedure of trial in the House of Lords, though the House of Lords is the court in both cases. Charges were brought by the House of Commons, not a grand jury. Additionally, while in normal cases the House of Lords tried peers only for felonies or treason, in impeachments the charges could include felonies, treason and misdemeanours. The case directly came before the House of Lords, rather than being referred to it by a writ of certiorari. The Lord High Steward presided only if a peer was charged with high treason; otherwise, the Lord Chancellor presided. Other procedures in trials of impeachment were similar, however, to trials before the House of Lords: at the conclusion of the trial, the spiritual peers withdrew, and the temporal Lords gave their votes on their honour. The last impeachment was that of Henry Dundas, 1st Viscount Melville, in 1806 for misappropriating public money (he was acquitted). Since then, impeachment has become an obsolete procedure in the United Kingdom.
The ultimate appeal was to the King or Queen personally, and the right to personally petition the sovereign called "access to the sovereign" is one of the traditional privileges of peerage that U.K. nobles held:
according to Sir William Blackstone in 1765, "it is usually looked upon to be the right of each particular peer of the realm, to demand an audience of the King, and to lay before him, with decency and respect, such matters as he shall judge of importance to the public weal." The privilege of access is no longer exercised[.]
In the time period from 1499 to 1901 there were 30 convictions of peers by the House of Lords that were not excused by peer privilege (there were no other convictions after 1901), all of which except a 3 month sentence in 1901, were death sentences. Seven of those convicted were pardoned by the monarch (most recently in 1746). Two peers had a death sentences commuted by the monarch to imprisonment in 1603. Two more escaped before being executed in 1716, and one died of other causes before he could be executed in 1589. Only three death sentences were carried out following House of Lords convictions since the last royal pardon in 1746. (There were five other House of Lords convictions for offenses other than murder or treason prior to 1841 and punishable by a sentence other than death, but in all five of those cases, the "privilege of peerage" was used to vacate any punishment for the crime of conviction because it was a first offense, so no appellate review or royal pardon or commutation was necessary.)
Thus, 30% of peers convicted and sentenced to death by the House of Lords were pardoned or had their sentences commuted by the monarch.
There was no forum outside of the House of Lords other than a royal pardon to which an appeal was available.
Incidentally, 30 convictions of offenses limited to murder and treason in this time period isn't vanishingly low. Before the recent contraction of the House of Lords, it had about 730 peers at any one time, so in the 449 year period for which we have records, there were about 328,000 person years of potential defendants, with a conviction for murder or treason every 11,000 or so person years. This is quite high by modern standard for an exceedingly privileged and carefully socialized group of people who didn't have access to firearms for most of that time period.