I agree re the fault symptoms of a bulb.
On the Coupe when the front wing bulb failed the remaining pair worked but went at a higher rate I believe.
Maybe a late thought for this situation but are you certain you have not had mice in there having a random lunch around the cabling ? Any signs at all.
I have two of those electric squealing things plugged in because of concern around this. We are just up the hill from the River Test and they have been seen around various back gardens etc.
