A109E Electrics
Nothing to do with water, I'm afraid - that would have been too easy!
No, after some serious dismantling and accessing wiring looms in different places, the problem has been chased down to a breakdown in either four or five adjacent pins in a very big block connector (120 pins) on the right hand side above the aft electrics bay.
Very hard to get at, harder still to detect and there's no telling yet how long it'll take to get it sorted.