Well gentlemen, I thought the most likely thing would be a dodgy feed, if there wasn't enough leccy geting through then the lights would take it all and the fuel gauge would get nothing. (Because the rear lights are on the same fuse as the gauges, don't know why).
So I was very pleased to find that the fuse in question (No 5 I think it was) was all corroded, maybe some water had got in there some time (quite often i expect!). So I cleaned it all up nicely and it made no difference.
Next I went to the lights, and the problem turned out to be a poor earth in the left hand stop light unit.
I cannot quite work out why a poor earth would cause this problem, cos surely that meant the light wasn't using much power - it was a bit dim - but anyway that did the trick.
btw I like the new pictures mr grunt / mr slob