Good question
The issue is that the law states that the owner is responsible for ensuring that the maintenance is done correctly, even though they may not be able to check the quality of the work.
Personally I was stitched up by an organisation that charged me for work they failed to do correctly. They tried to invoice for the hours theyspent learning to do a simple task they said they knew how to do. I was invoiced for 37 man hours for a job that would take 10 hours max and then did it wrong.
I paid for it to be rectified later at the annual. Yet the organisation has all the approvals!!