It seems this is in fact a common issue all leading to ....the switch! I might have to bite the bullet and buy a second hand seat switch for £50.
Thanks for all your help here but I probably won’t be...removing hardtop, removing seat, undoing seat to look if self fix can be made, then ordering parts...anytime soon. As stated above it does feel looser so I’m pretty sure I can point at the switch as the source of the fault. Still confused about the voltage drop, but hey.
I’ll update this with some pics when I tackle the job!