Over the last two years I have asked several ATC units from OCAS to check our Mode C and the result has been OK. However, during a recent transit of Solent CTA at 3000', we were asked to switch off our transponder because they said it was reading 200' low. We later asked Bournemouth and Bristol Radars to check it and they said it was within limits. Calculations based on the displayed PA and the QNH confirm that it is slightly low but within limits.
As a Permit aircraft it doesn't usually go anywhere near expensive avionics facilities, how else can one verify the accuracy of Mode C?