Ged did you ever get to the bottom of this ?.
Steve

I think so, I've had problems with fuel contamination in the past and I think that some kind person had put something in the fuel, the fuel cap isn't great so I've put a padlock on it. When I cleaned the fuel out I burned some in a pan and there was a residue, so I assume there is something that clogs a valve for a while stopping it working then over time evaporates and allows the pump to work again???
Or it could be none of the above the first 2 or 3 pumps used were delphi, the last one that's still working was, dare I say it, a britpart, only put on 'cos we'd got one handy and it was a switch in desperation