the starter gets hot after a run (usualy on a type 4 if the heaters are off so they vent out onto it) this causes high resistance that saps the power from the aged wirng and will not let it turn over, it would still however bump start or when the starter is cold work fine, a relay (if fitted) just bypasses the aged wiring and "boosts" the output to the starter.
your relay (if fitted) could be in the engine bay or underneath - try looking at the starter and see if there are any wires that look non standard going to a small cube.
Mine did this for months until I fitted the 'hot start relay', only done it once since and that turned out to be where some idiot (guess who?) hadn't screwed the earth in properly.
mine did it with the hot start relay fitted (by p/o) so I removed it and re did the connections and cleaned them and touch wood it's been ok since without one.
Cheers Malc - not convinced I have one fitted having a hunt around. RAC guy about 20 minutes away now.
We are running. RAC guy reckoned dodgy starter because it started randomly while they were poking about. Hot start issue sounds favorite.