But then what guarantee is there that a pilot will not just simply get a page of entries signed off, then edit at will to make the hours accrued bigger? I would have thought that if the CAA, or any authority, received a logbook full of correction fluid it would have raised suspicions.
I don't think there are any guarantees. The system relies on the participants being honest! There's a specific offence under the ANO which (I think) prohibits making misleading statements about hours to obtain qualifications, but this must be difficult to police.
There are ways the CAA could check your entries, e.g. calling the airports you say you've visited and asking to see ATC records, or cross checking your log against the tech logs of the aircraft you've flown. Very difficult if it's your own aircraft or if you fly to unlicensed fields where there are no ATC records.
Not sure how often this happens in practise, but I've read stories of pilots with "parker pen" flying hours at all levels up to and including airlines! e.g.
Pilot suspended over logbook 'fake' | UK news | The Guardian