Hi , not quite sure what fixed the problem, we simply cleaned all the fuel lines right through to the fuel tank and it now appears ok, been nearly 5 days without any problem.
We did think that fuel was leaking back down the fuel line from the fuel pump, perhaps the fuel pump diaphram may have been leaking and did consider putting a non return valve in to help this issue, which we will do if the problem resurfaces, but for now it's just a lovely feeling to fire up the old twin cam and go to work every day.