I'm not sure what the score is basically. As far as I can work out I think that whoever has had the bike before me has tried the fuel tap vacuum bypass modification, as explained here: http://www.akhara.com/nc30/fueltapmod/index.html but they've not done it properly. When I took my fuel tap apart there was just the large and smaller diaphragms and the small piece of metal that goes between them that they both seat in to. According to the guide on the website above, there should be another plastic disc present, which I don't have, and the spring is also missing.
It seems strange that someone would try the vacuum bypass mod and then still keep the vacuum pipe connected. The only thing I can think is that they've tried to do it and simply done it incorrectly. I've ordered a fuel tap repair kit from NRP carbs in Manchester, and that gives me a new spring an diaphragms, but no plastic disc piece (pictured here second from left at the bottom:
. My problem is that I've not got the correct parts to make the tap function with the vacuum, or without it. Does anyone know where I can get hold of that plastic disc or have any suggestion for getting my fuel tap to function properly either with or without the vacuum? I can take some pictures of the guts of the tap that I've got if that's any help.
Surely you'd want it done properly? The more I investigate my bike, the more I come to realize that the bloke that had it before me either didn't care or just had no mechanical sympathy 
I know if I'm going to attempt work on it and I rech a point where I'm not able to do something, I'll generally ask for help so that it's not in a state. Oh well.