Hersham Boy,
The CAA Library at Gatwick has EVERY AAIB report since 1950. Ceratinly makes interesting reading if you're interested in such things......Nothing has changed since 1950, pilots are still making silly mistakes!
Pulse 1,
The regulations can be read here
http://www.legislation.hmso.gov.uk/si/si1996/Uksi_19962798_en_1.htm
A lot of accidents that don't appear are either due to an ongoing investigation, (SSD - such as the Cherokee Arrow over Lambourn) or the pilot 'forgets' to report it, either through ignorance or because of other motives, such as fear of prosecution.
Another thing that always amazes me is how different the AAIB report is to the 'actual' event. A report a few years ago of a minor accident resulting in a burnt out aeroplane bore absolutely no resemblance to the accident I witnessed....I wonder why?