I'm both very happy and very sad to announce that I think I may have fixed this fault! Sad, because it seems to be something I have caused. In desperation, I had the idea to remove all the HT leads to check their resistance cold and then hot. In doing this I noticed that the orange part of the distributor cap was badly cracked in about 4 places. I have never thought to check this before because it was a brand new cap which I fitted way back when the car would not start at all (fault which was definitely the coil pack). But thinking about it now, the fault only started occurring after I fitted this cap, which is an aftermarket no-make that came with a new distributor I got from work (ECP), which incidentally was rubbish and could not be fitted owing to it being machined incorrectly! I swapped the Hall Sensor from the new distributor and thought I might as well use the new cap as well.(Mistake!)I have fitted a new Bosch cap and rotor arm and car runs perfectly and has idled now for 2 90 minute stints without cutting out. The moral is, do not buy cheap parts if you can help it. I would like to thank everyone who has followed this thread and given their input, especially Crasher.