ACP, you could also write to the Safety Data Department at the CAA, Aviation House, Gatwick, RH6 0YR. They'll be able to give you a load of info on individual level busts.
A common cause of level busts is that SID's (Standard Instrument Departures) in the UK, with the exception of Birmingham, finish at an altitude (based on QNH, ambient atmospheric pressure). Onward clearances are usually to a flight level (based on standard altimeter pressure setting 1013 mb). If a crew forget to change altimeter settings when they receive their onward clearance, and the mistake is not subsequently detected, they may end up levelling off at, say, 15,000' amsl (on the QNH). If QNH is less than 1013 this will result in their having levelled off higher than FL150. If the pressure difference is more than (approximately) 7 mb, the altitude difference will be greater than 200', resulting in a level bust.
For this reason ATC will shortly start issuing onward clearance which, when clearing from an altitude through Transition Altitude to a Flight Level, using the following phraseology:-
"XYZ123, climb Flight Level 150, set altimeters standard 1013."
Hope this is all clear enough!