Ment to update is last week.
Checked the rear light cluster in the rain, and yes: water on the inside of the bulb holder. Dried it out and cleaned up the corroded connectors. Plugged it back in and the problem is gone. Been about 10 days now of wet weather and all still working fine.
Next task was obviously to remove the rear lense and replace the seals. Problem is I think a previous owner has tried to fix the water ingress by using a silicone sealant between the lense and the bodywork, there was no way that rear lense was coming out!
Looks like I'll just have to keep the connections cleaned up every so often. Thanks for pointing me in the right direction
