Just a note from the UK perspective:
MATS 1 GEN 4.5 : "where a controller can acertain from the code allocation plan that an observed Mode A code has been assigned by a unit capable of validating the code, and has not been notified that the code is corrupt, then that code may be deemed validated"
and:
MATS 1 GEN 9.2.3: "A Mode C readout can be assumed to have been verified if it is associated with a deemed validated Mode A code."
So basically if you have been transferred from an adjacent centre to London Control, we can accept the current pairing as sufficient. However I think there may be an exception, but it may have gone with the North Sea changes..........
