I have a lodger living with me at home. I got my Landlord's gas safety check done but a bit late (it was a day after the last one expired). I just read that if anything happens they will look at your gas safety record history. Is there anything I can do about it now? Will I get in trouble for this?
As @DavidSupportsMonica says, you can't go back in time to fix the problem.
As a rule regulators are more interested in obtaining compliance than throwing the book. There are no sentencing guidelines for health and safety offences, but most convictions lead to a fine. However HSE Policy is that enforcement should be proportionate to the degree of non-compliance and the level of risk created.
5.2 In our dealings with duty holders, we will ensure that our enforcement action is proportionate to the health and safety risks and to the seriousness of any breach of the law. This includes any actual or potential harm arising from any breach, and the economic impact of the action taken.
5.4 Applying the principle of proportionality means that our inspectors should take particular account of how far duty holders have fallen short of what the law requires and the extent of the risks created
An annual inspection that was overdue by one day is a very minor failing, and the principle of proportionality means that you are most likely to get verbal advice not to let it happen again.
You say "if anything happens", presumably meaning if there is an actual incident or serious risk, such as a fire or CO poisoning. Its true that in such a case they will certainly want to look at your records, but the proportionality rule still applies. They would need to be able to make a causal link between your failing and the actual incident to make any more of it, which wouldn't be the case here.
You can't go back in time and have the property inspected so it will have been covered on the missing day. Thus, there's nothing substantive you can do to fill the gap.
The inspecting authority may not notice. If they do notice, be polite and agree that the gas safety check was indeed late, but point out only one day passed before the property was rectified, and that nothing happened on the uncertified day.
It's possible the inspecting authority will have the power to impose a fine or penalty for a certification gap. Whether they have that power, and whether they apply it, is not within your control.