Thanks guys! :-)

I'll check all these over the weekend.

What makes tracing this problem especially annoying is that it's intermittant. It was fine again this morning, but judging from the past few days, it seems to play up mostly in the evenings, especially when I've just got home, but it's usually fine when I leave work. This makes me think more of the connection block, as perhaps the heat generated by having the tail lights switched on is disrupting an already poor / damaged circuit.....?

If it turns out to be the switch on the gearbox, does the gearbox need to be removed in order to replace it?