hear where your coming from, but if an innocent custard has an illegally installed system, I have gone in and followed the advice on the tb to cover my bum, and its cost the owner almost as much for labour as a normal boiler swap, if not a bit more, as I am covering the whole setup as well once youve signed. done it once, the other times i was asked they didnt like the quote🙂
Having been asked to sign off illegals on other occassions, I reported them to gas safe and trading standards, to have no effect at all as the customers were happy and not complaining, in one case building control went in to look at a big refurb, but were told all the gas work was original and not touched, depite them having asked me to sign of new lpg supplies and cookers etc!!! It wont stop until they stop selling kit to the illegals!