This failure of traffic to follow the requirements of the airfield is a constant problem. If an overhead join is required then EVERYBODY should join overhead and make the appropriate calls. More often than not home based aircraft seem to think that this doesn't apply to them at their own airfield and they just join crosswind, downwind or straight to base. I have been involved in perilous situations at Nottingham and Welshpool by local traffic just joining downwind despite our making the join properly and putting out the correct radio calls.