OK I think Iv'e got it sorted.
I pulled the tank out and found that the fuel line inside the tank had stretched and the clunk was resting at the back of the inside of the tank.
This was causing the line to kink and restrict the flow of fuel. I'm not sure about the air bubbles but Iv'e replaced all the fuel lines as a precaution.
This should have cured the problem, but I will have to wait till my next visit to the field.
Thank you for the advice, it is much appreciated.
Don