Cannot help with any reference, but if you are still in the TMA when you reach 1000' below you SHOULD be on Terminal QNH I would have thought. Once outside Terminal airspace area would be logical. There seems little point in snagging the transponders since you all appear to possibly be flying at the wrong altitude! I thought also that Transponders used their own 1013 independent module for readouts and that ATC converted the reading to QNH if required?