Musket90 - Thanks. That seems to concur with what I have found out in the last couple of days. A clearance to land from ATC can be taken as acceptance by the Aerodrome Fire Authority for the aircraft at the reduced category available.
Our Ops manual states what category we may land with, so we are aware what we can accept but final say (except for emergencies as always) does appear to lie with the aerodrome authority due to the licencing implications.
I ask because we ended up diverting following a temporary depletion and there has been considerable discussion about the rights/ wrongs since!