Did you check to see what the power source for that instrument was? Not all are electric. Did you check to see if a circuit breaker was out?
I can't speak for UK regulation, but generally speaking, if it's installed, it's got to work. If it doesn't work, it needs to be deactivated and placarded inoperative, so long as it's not required for the kind of operation in use (daytime, VFR, for example).