Mike Jenvey,
Just to follow up your comments with a bit more information; (% are approximate)
Missed read-back error (pilot read back is wrong and is audible, ATCO fails to hear and correct the error) accounts for about 10% of level busts,
Incorrect read back by pilot accounts for about 8% of level busts
Read back by wrong aircraft (taking a call intended for another pilot) accounts for about 6% of level busts
Poor manual handling, aircraft technical problem and altimeter setting error each account for about 5% of level busts.