Inky: Note that the tap you need is NOT an M5, it is in fact an M6.5 - a very odd animal indeed! I don't have one, as I epoxy the feed housing into the section of the pen. Probably just me excusing myself for being a tight git! I think I can say with some confidence that you won't get that tap as a standard piece anywhere on the planet - Mike Redburn's (Silverpenparts) price is reasonable but will almost certainly attract duties and an exorbitant admin fee from Royal Mail, so budget that in too.
Paul: You need a few taps/dies, as follows: Manufacturer-specific tap to thread the feed housing into the section. Bock, Jowo and Schmidt all use a sightly different standard - what a surprise. (I don't do this, I just epoxy the feed housing into the section, which is essential in any case if you are using the barrel of the pen as your ink reservoir).
Tap and die for the section-to barrel thread. Commonly an M9 or M10 by 0.75 or 1.0 (relatively easy and cheap to come by).
Tap and die for Cap-to-Barrel thread. Boy - now HERE'S a can o' worms! Commonly and M12 or M14, BUT ... commonly a triple-start thread. So ... M12 or M14 by 0.8 x 3. Again, almost impossible to get, you can try silverpenparts, Richard Greenwald, Indy-Pen-Dance or any of the other boutique pen makers' shops, or you could wait for a group buy thread on IAP (which happen more frequently than you might think). You need a matching tap and die, and the going rate seems to be about $150 ex shipping/duties/Royal Mail kiss-my-ass fees.
OR, you can do what I do - use an M12 or M14 by 0.75 single-start (i.e. bog-standard) thread, which means that you have to turn the cap once or twice more to fully close it. Oh, the HORROR! Interestingly there are a few major pen manufacturers who stick with single-start threads (and not a few Penmakers Guild makers too) so you would be in good company - although looked down upon by the aficionados on IAP.
And Brian thinks that kits are a hornet's nest - oh the wonders of kitless that await the unwary soul! 
