The problem seems to have gone away, but hasn't been hot for the last couple of weeks (nice 25 degrees c).
I agree with you Noel the culprit must be the fuel pump. It is brand new but It doesn't appear to be "branded" so I assume it is "after market". I am going to try putting the spring out of a diesel lift pump in it and then wait until next summer to see if it is cured.
Thank you all very much for your help
Cheers
Alan