So I got the bike back to mine, stripped all the fairing off, checked and cleaned connections/sensor plug on right hand side and at cluster, had a new chain, sprocket, front bearing, fork seals, service and checked valves, put it all back together and the problem is solved for now anyway.
The battery was new this week and I fitted it, so i don't think that was the issue in the first place, the bike was stored outside under a lean-to with a decent cover on in South London
So out of direct rain but exposed to atmosphere
None of the connections appeared corroded or had surface oxidation as far as I could tell.
So difficult to ascertain if it was a simple clean and reconnect of the system, or 48 hrs in a dry slightly warmer workshop(15 degrees C maybe) that has solved it.
Hopefully won't reappear, was a bizarre one.
Thanks for the spare cluster offer, hopefully not needed at the moment.