HURRAH! I finally fixed the problem for £2.50 !!! Cyber pints all round.
I think it was SoCal that directed me to check for poor earths so thanks for that. I DID check all the earthing points and they were all good wire with clean connections on shiny metal ... and yet....
Despite an apparently excellent, tight, clean connection twixt the big fat earth from the harness behind the engine to the firewall this proved to be the cause. I have no idea why this was faulty but I fixed it by placing a nine inch length of braided earthing strap (8mm eyelet type)) underneath the harness lead connection on the bulkhead and connected this to a stainless M8 screw through an existing hole in the top of the firewall on the drivers's side (passenger side for those of you who drive on the wrong side of the world).
Hey presto! Instant smooth running. I have no idea why putting a booster cable from the engine to the neg battery terminal didn't show this up but ...
Anyhow ... this sorted the problem and instantly revealed another ... my headlights no longer popped up!!! Surprisingly for me I sorted this out quite quickly too - seem that the connections in the lifter motor were a bit sloppy causing the relay that slots into them to cause poor connections. It appears that the poor idling compensated for this by jiggling the connections sufficiently to operate the motor - I figured this out when I flicked the relay with my finger and the lights immediatly flipped. Quick strip down and judicious use of a pair of pliers to tighten the female connections fixed this immediately.
I'm ready for the next problem now.