Success! Removed the live supply to everything by disconnecting the battery. Ran continuity test from T connector to fuse holder and that worked. What I didn't do was the same for the black cable to ground. Just did that now and all good. Stupid moment of it not occurring to me that for the most part everything is earthed to complete a closed circuit to function at all. What did surprise me though is that only by disconnecting the battery then reconnecting everything was back to normal. Now wondering if shorting the circuit tripped something in a relay or some other logic box somewhere. Removing and then renergising the circuit performed some 'reset'. No idea but I'll take it.