spent about 4 hours today getting the dash out examined the heater externally in detail and found sod-all wrong with it.
bugger.
hot water circulates through it, air comes in and goes out, (and I did fix the recirculating air control, vacuum pipe had come off) but no hot air. Arse and arse.
so... I suppose the next thing is on a day when I have more time, pull the heater and take it apart and see what's wrong.
Unless someone has a known-to-work disco 200-type heater which they'd part with for a reasonable sum...?