Thanks everyone for the replies.
SlotDriller – yep, hadn't thought of that, my Dads ancient bit and brace has a 2 jaw chuck but its a bit on the heafty size for the M2.5 tap I want to use.
Mike – I did see your website with the 4 jaw – very nice, but definetly outside my skills limit .
I suppose it might be easer to modify a new or boot sale special Eclipse type – but I wondered about it's accuracy – although if the flats aren't ground 100% accurately (which sounds like they're not !), it's a bit academic. I'll have a look at my collection of taps at the weekensd – meantime, if anyone from ARC or Chronos wants to introduce them, it does look like there's a couple of buyers all lined up for you !
W.