b) there is a fault on the fixed wiring. I've tripped all the MCB's
to try to isolate the circuit with the fault but this has not helped
as even with all the MCB's to off the RCD will ot reset.

Disconnect the load bus bar and load neutral from the RCD. If it doesn't
reset now, then the RCD is definitely off to silicon heaven. As it trips
with all MCBs off, I suspect this is the likely cause, although it is by no
means guaranteed.

Christian.