Hi JonathanJ Thanks for your help.

We've checked the fuses behind the glovebox and they all seem to be fine? We've also taken a multimeter to the switch terminals and there is definitely 12 volts coming from what appears to be the ignition on / off circuit so it seems power is getting to the switch?

I've also had a wiggle of the wire looms behind the dash and that doesn't seem to be having any effect?

As i recall as the fault was manifesting itself the dash lights would work intermittently until finally after a few flickers as i went over some road surface bumps they eventually died which implies a dodgy wiring connection?

I'm completely baffled? Online searches produce results which implies it's a reasonably common fault but no-one has given a solution?