OP
MarkieSparkie
I've moved most of the visible wires away. There's one area where some lighting wiring shares an entry point with pipe work and has been plugged with expanding foam, I'm guessing that's a possible hot spot !!
Some expanding foams (and Gripfill) have an adverse reaction with PVC cable insulation causing it to breakdown.
I had a situation a few years ago where an outgoing lighting cable shared an access hole with the incoming overhead supply cables. This hole had been filled with expanding foam causing breakdown of the cable insulation allowing leakage from the incoming line to power the lighting circuit. The first indication of the problem was when the switch cable was about to be cut to length and the cutter blades switched the lights ON even though the installation was "safely isolated" at the DB main switch.