Urrgh I wouldn't touch that if I was myself... I've got plenty of challenges in a single carburettor!
Sounds like dirt, but then again I'm not an expert.
I know what I would do... I would call Roy, give him a good and detailed problem description, and then ask him politely for his opinion on the matters. Since he's across the sea for me, I can't show him my car... but hey Antony, you are in the same country!
Cheers,
Anders