At s23, the court cites Chief Justice Wilmot's decision in the 1769 case Thornton v Cruther & others, which it mentions is "unreported."
So how did the court become familiar with the case, much less the content of its reasoning?
The Law Reports at Abingdon Rural District Council v. O'Gorman  2 QB 811, 820 read as follows:
[T]he judges held that in order to "impound or otherwise secure" the distress on the premises the landlord had to move the goods into a fit and convenient place on the premises … If there was no fit and convenient place on the premises, the landlord had to remove the goods out of the house. It was so held in 1769 by Wilmot C.J. in Thornton v. Cruther¹ and in 1774 by Lord Mansfield in Washborn v. Black,² and accepted in 1846 by Parke C.B. in Woods v. Durrant,³ and by Pollock C.B. in Smith v. Ashforth.⁴
¹ (1769) Unreported (mentioned by Lord Mansfield in Washborn v. Black (1774) 11 East 405, n. (a)).
The reference to East is available at the Internet Archive at Reports of cases argued and determined in the Court of King's Bench, vol. xi (1805-18), 405:
Sittings at Westminster after Michaelmas, 1774. Buller J.'s MS. … Mr. Dunning insisted that he was a trespasser; for he ought either to have put the goods all into one room, and kept possession of that only, or to have removed the goods out of the house. And he cited a case of Thornton v. Cruther and Others, C.B. Mich. 9 G. 3. before Lord Chief Justice Wilmot, where it was so holden.—Lord Mansfield C.J. said that the strict law was so …
The report draws on a manuscript of Buller J, who became a judge of the Court of King's Bench in 1778, describing a hearing before Mansfield CJ in 1774. Perhaps Buller was present as counsel in that or another case, and making notes for his own future reference. Counsel in the hearing before Mansfield CJ cited an unreported decision of Wilmot CJ on the Common Bench, Michaelmas term, in the ninth year of the reign of George III (1769). Mansfield CJ accepted it as an accurate statement of the law.
It is probably unknown whether Buller or East checked the citation against the records of the Court of Common Pleas available to them in the late 18th and early 19th century, such as the plea rolls. However, in this era, historical court records typically did not include judge's reasons for decision, which were announced orally in court. These reasons could be noted down by barristers, referred to in subsequent cases if the barrister was considered trustworthy, and published by the authors of the nominate reports for wider legal education.