I took Marque's advice and purchased some Plus Gas. I have been spraying the problem thread/stud all week as advised. Attempted to replace the bypass valve this afternoon and success
the housing came off no problem and the bypass valve has been renewed.
Out of interest the car picks up a lot better with the new valve fitted, especially at lower or mid throttle; before I was experiencing the yo yo effect which could only be overcome with full throttle.
Thanks for the advice