Hi Guys - Thanks for the info and apologies for lack of response but i've been waiting on new pulleys and timing belt to arrive, which they did yesterday. So brief update - I used the info above and cobbled together a load of wires and croc. clips together with a 20v PSU and a 10v wall wart and I managed to get something approaching control but not full speed even though the setting said it should be 3000 rpm. So did a bit of chinglish, interpretive reading and adjusted the source AC power as we are 240v in the UK which seemed to help, then upped the top speed to 4500rpm and this got me back to about the 3000rpm of the specs. When the motor pulley arrived it got installed - now ratio was 22-42 so chuck / spindle speed should top out at around 1650 odd rpm.
I've utilised a little project box to house the 20v PSU and a new 10v adjustable buck converter that goes through the analogue control and also powers a digital LED tacho that reads spindle speed via magnet and Hall device - that was a trial, wired it up 3 times but it wouldn't function - it turns out the Hall device is particular about the magnet polarity, once that changed i've got rpm read out. I've just got to 'glue' the magnet to the spindle as although its a strong 'lil devil, i'm fairly sure it won't stick around at vmax with vibration added!

Its been a journey but the lathe is now back in service and I'm quite proud of the changes i've made and its got a bit more power behind it, although its capabilities will probably still outstretch mine by a long way.