Dietmar,
Cool, thanks for the reply. Last night I got a copy of the wiring diagram of the 160TCL thanks to a kind gentleman who is retrofitting a 125TCL but for some reason has the 160 manual! The diagram indeed confirms that the two yellow wires are a thermal cutoff for the spindle. I may well wire it in as it is there but will leave it disconnected until everything else is working.
I will try controlling the spindle with relays first and report here how I get on. The wiring diagram shows how the relays on the existing board work so I will try and replicate those first. The guy I got the manual off had problems trying to control the spindle using a CNC4PC breakout board which doesn't inspire confidence but I have ordered it now so will try it when it arrives.
Typically, now that I have the wiring diagram it doesn't look that hard to wire in the original controller but as the Geckos have arrived, I may as well use them now!
Also, interestingly, the boxford 190VMC mill seems to use the same controller as it is shown in the manual as used for both and that is my next project after the lathe is finished!
Simon