Problem is solved. I am posting for the search archive.

Brief investigation revealed that in it's resting position the pedal shaft was not
striking the plunger on the switch with sufficient force to break the circuit. Unsure
what the recommended fix would be, but since my disco has the later monolithic pedal
assembly which needs to be evacuated through the engine bay, I opted for a hack.

A clothes button glued to the end of the plunger sorted it initially. After installing
a new booster a triple layer of bread bag clips needed to be taped to the pedal shaft. Held
together no worries for the inspection, we'll see how long it lasts:-)