OK, thank you for the many answers and thoughts about what to do.
I am going to take out the filter at the weekend and put in a new one. I had already put a new sponge filter on the regulator. But i do not think I have had the real answer as to why it happens after half an hour and at 3000ft as against 1500ft --- or am I missing something in the previous explanations. The AH has been tested on the bench and the suction gauge reads 5.
As for the comments about homebuilts --- they are often the only way some people can afford to fly. Not renting a spamcam at £200 ph !! I do virtually all my own maintence that saves me at least £5,000 a year ? I think the Vans RV6 is one of the best to fly and most efficient with a Lycoming. They are also very docile to fly compared to others. I think 8,000 in the world and 250+ in the UK says something.