Ian,
The drawbar thread for the W20 collet is a 19.6mm (although I have also seen 19.7mm mentioned) diameter by 1.66mm pitch. I found that this was impossible to gear for on a metric lathe so resorted to cutting the threads on an imperial change wheel lathe by, as mentioned by DC31K; the easy way is to convert it to an imperial pitch = 15.246tpi
This was as near to 16tpi so, on looking up the change wheels need for 16 tpi, I substituted a 51T wheel in place of a 48T which gave the ~5% difference needed to get to 15.25tpi. The thread is also a buttress 45/5 degree.
Enjoy your machine – they are a delight and if you have been really lucky with getting the accessories, like the high speed head, complex quartering table, vertical slotting attachment, the delightful dividing head and the really rare offsetable rotary table, then there is not much you cannot produce on it.