Precisely the same thing happened to me at Old Warden a while ago on a display day. The arrival brief included a requirement for an overhead join and a specific arrival slot....
I planned things carefully, arrive in the overhead at the right time and everything was looking fine. Then a "Golf Alfa Life History Sir, over" character in a Cherokee 180 from Thruxton announced that he was joining downwind and cut in front of me. I was then forced to follow his airliner circuit halfway to the North Sea; this delayed the landing of the Shuttlewoth Lysander...
I suppose I could have carved inside the idiot and landed as I'd planned; however, that could have caused yet more angst. The pilot of the Lysander was very polite and held off; we later chatted about it and he agreed that the poor airmanship of the other aircraft and his failure to follow the published arrival procedures were wholly to blame.
The trouble is that 'conventional' joins are being displaced by downwind joins, crosswind joins by pilots who barge their way into the aerodrome circuit rather than fitting in with other traffic - and, of course, those who insist on attempting the American 45 deg entry into downwind which is NOT a normal UK procedure. Add to all that the HUGE circuits often flown 'for noise abatement' and it's hardly surprising that aerdrome circuit procedure standards are slipping.