There some very strange opinions posted here. For example - Why because there's a maintenance organisation which is ALLEGED to be less than excellent should aviators "avoid Elstree"? No-one is charged for maintenance at Elstree just because they land there, buy fuel or indeed park an aeroplane there! The runway's fine and services generally not expensive.
Next - when one gives the logbooks to a maintenance organisation and instructs it to carry out a 50 hour, annual or whatever, there exists a contract. If the case described here is true it's as plain as the essential parts of a male dog, that the MO failed to honour that contract, by failing to carry out the work specified on the schedule laid down by the relevant authority - not once but consistently over an extended period. So - sue them, the company and the relevant officers of that company for breach of contract, fiduciary duty etc, taking moneys under false pretences etc. You may find that the Company has no resident, full-time engineer or indeed one listed as a Director; this is something any a/c owner should know about their MO. In any case, whoever the engineer who "signed off" the annuals etc should also be sued (they have to put the CAA Eng No. in the logs so you know who they are.) Personal legal actions tend to attract the attention of the recipient!
The only way to stop this malpractice is to attack the perpetrators - legally. Put them out of business!
Or I suppose you could accept a small payment and let some other poor sod who doesn't read forums (and most don't) get ripped off…… or perhaps hurt when his a/c fails while airborne. I wouldn't want that on my conscience.