This place is fantastic http://www.tapdie.com/ I picked up a G1/4" tap, m4 tap and handle from them a little while ago and they were really helpful. You'll also need an 11.8mm drill bit to make the right size hole (they sell those as well).
http://en.wikipedia.org/wiki/British_standard_pipe_thread Wiki is your friend: G = external+internal parallel (ISO 228) R = external taper (ISO 7) Rp = internal parallel (ISO 7) Rc = internal taper (ISO 7)
Thanks, I never knew that and I've used them, though only the dies on pipe. 1/2"BSP (Parallel) taps you may find at a Plumbers' Merchant.
Or a bumper bundle from Amazon for £19. BSP = British Standard Pipe, = BSPP = British Standard Parallel Pipe, = BSPF = British Standard Parallel Fixed = G BSTP = British Standard Taper Pipe = BSPT = BS Pipe Taper, = BS Conical Thread.
Well, i've got one now.. Coolmiester was nice enought to lend me his... It also seems that www.cromwell.co.uk sell them too!