For ball valve fuel taps try:
http://www.screwfix.com/prods/23550/Plumbing/Gas-Fittings/Mini-Lever-Ball-Valve-8mm
They state 8mm, but's that the od of the pipe that fits to the fittings that come with it. Remove the nuts and olive and you will find the body it threaded 1/4 BSP. I removed the filters off the old taps and glued them into the new tap's shouldered end with araldite rapide which I have found to be fuel proof when cured (can be used as an emergency repair to fuel tank leaks). The taps then screw straight into the tank using your favourite sealing washer / method. The standard Vincent fuel pipe and fitting connect directly to the other end.
You may need to prise off the yellow cover to remove the lever (held on with a screw under the cover) to swap it round 180 degrees so it points down and not up when the fuel tap is open. I left the yellow covers off, or you could paint them black.
If you use carbs with filters built into them, then you could risk not fitting filters to the taps, or fit inline filters.
The taps are small, designed for fuels, no adaptors needed, easy to use, cheaper than the standard type fuel taps, chrome plated and do not leak.
For a photo of one fitted got to Photos - Misc - Misc 2010 - modifications, or if the link works:
http://www.vincentownersclub.co.uk/photos/showphoto.php?photo=6422&title=img-1277&cat=767
Cheers,
Simon.