Note to self, write in more detail...! Any work carried out on the aircraft (and its components) and parts fitted should be recorded in the aircraft logs and all paperwork kept on file - ADs, checks, anything.
But, you're absolutely correct that this isn't always the case - one reason why pre-purchase surveys include log book studying but with the proviso that people only write in them what they want others to see. The unscrupulous may not include the fact that they have fitted bogus parts, not shockload inspected the engine after a prop strike etc.
One of the facts of life is that the CAA surveyors concentrate more on the paperwork side of things than the actual physical state of the aircraft (although they do love checking placards during surveys!). A number of engineers have had their licences revoked or suspended and maintenance organisations had their approvals cancelled because of paperwork irregularities. Part-M brings even more paperwork, particularly during the organisation approval process and those that struggled to maintain their M3 approvals have little chance of surviving the path to Subpart F and G....
So as my colleagues have stated, we do what we feel we have to do (ask the CAA and they'll say non-commitally 'interpret the rules as you see fit') to keep the aircraft flying safely, our customers happy and the lawyers away from the door!