Just a quick update to let you all know the outcome.

I ended up buying a serial to RS-485 adapter (less than £3 on fleabay) and downloaded a plugin to ebable Mach3 to control my spindle directly.

The new spindle is fitted and running and I was originally using a 5 gallon drum for a water reservoir but this didn't stop the temperature hitting the mid forties centigrade during the hot Easter weekend. I have to point out my workshop is upstairs and the room temperature was in the low thirties. As downstairs is always a lot cooler and having plenty of plastic pipe to hand I simply added about 15 metres of pipe to the outet and ran this downstairs then bak to the tank which solved the cooling problem.