How can I fix Potterton powermax HE?

Seem to have same problem as every other lucky owner of a potterton powermax HE 115. Keeps going into lock out code A 0.1, press reset it starts but goes straight into lock out. Have to top up pressure every few days. Do not seem to have a leak anywhere. Had people out to look at can't seem to find the problem. We have been told it might be the boler control assembly part number 5107535. Does anyone know any where that does cheap parts?