Rover 75 v6 petrol engine cuts out in very hot weather?

I have a 2004 rover 75 v6 petrol engine which cuts out on very hot days only after driving for about 45 minutes and won't restart until the weather cools down. I have had it checked by dealer and they replaced cam sensor which made no difference. I suspect it may be a fuel pump problem but with two pumps fitted which one? Again this only happens in very high temperatures, I don't have any problems on cooler days.Any ideas?