Thanks for the advice - I do have a small update.

After posting yesterday, and speaking to a mechanic friend, I desperately went in search of a vacuum leak as it appeared to be the only thing left that it could be. So, I removed and cleaned the throttle body (which was in pretty good nick actually), cleaned out the PCV valve (just in case) and then removed the one thing I hadn't disturbed until then - the MAP sensor which sits next to the Idle Control Valve on the inlet. It looked pretty dirty so i gave it a blast of electrical contact cleaner, dried it off and refitted it. Now this is the later 2000 1.25i engine which Haynes seem to have totally ingored in their manual despite it's quite obvious differences in the inlet side of things so I wasn't about to go blindly into removing the inlet manifold at this point.
Having re-assembled everything I went to start the car to show my neighbour exactly what the problem was - and, surprise, surprise, it idled and ran perfectly !!! I restarted it a couple of times to check that it really was ok, then went indoors pretty pleased with myself and texted my son to tell him it was fixed. Three hours later, just before my son was due in from work, I popped out to check it again - started it up and the fault is back, just like before

So, now we're thinking MAP sensor - obviously cleaning it had an effect on temporarily remedying the problem, but as soon as it get a bit dirty again its failing - does this sound plausible to those in the know ?
We've cleaned it out again this morning and got rid of the fault again, but on the second time of starting the fault is back.

Mind you, here we go again with another fifty quid part, so I'm just looking for someone to give me a steer that we are on the right track.

Thanks for all your assistance, it really is most appreciated.