right, story so far...
Definately needed new distributer, now fitted.
Definately needed new plugs, now fitted.
also changed air filter (old one was

) and cleaned throttle body.
Engine starts and runs fine, with no load, and is very smooth above 2000 rpm. BUT once it warms up it stutters badly and has no power at all - still revs freely above 2,000 but hesitates terribly under that - Cannot possibly use it on the road.
Seems to be some burnt oil out the exhaust which looks like it is coming from the crankcase breather system. Wasn't doing that before - so figure it is not the real problem.
Similiar problem on my Dad's car (Range Rover) turned out to a u/s Air Flow Meter, so I have got a second hand one off eBay to try and see if that sorts it. Should be here in a couple of days.
Any other ideas guys?? Thanks