Hi 741A,
Eurocontrol Ops Manual Part 1 states
Application of Vertical Separation Using Secondary Radar Mode C
Verification of the accuracy of Mode C- derived level information displayed to the controller shall be effected at least once by each suitably equipped ATC unit on initial contact with the aircraft concerned or, if not feasible, as soon as possible thereafter. The verification shall be effected by simultaneous comparison with altimeter derived level information received from the same aircraft by radiotelephony.
You would then see this as you fly through Redfa,Lamso, (ex London ATC) Toppa (ex Scottish ATC), WSR (ex Bremen ATC), ALS Tuska Grefi (ex Copenhagen ATC) and Departures from Brussels,Amsterdam ACC's etc. Just to name a few.
The reason I think is basically that we are running different Radar Systems to each other, therefore the first sector for each unit is responsible for verifying Mode C for the entire Unit.