Please feel free to mock me severely
After reading antonye,s reply I had a classic Doh!!! moment , ofcourse it was a pressure problem cant for the life of me think why i was over complicating things
Sure enough went out , pulled the breatherpipe off and was greeted by a rush of air into the tank , a quick blow through the breather valve confirmed its on back to front . Have'nt a clue how long its been like that , looking at the service history its last MOT was july last year and the previous owner only managed to amass 72 miles from then until november when it was SORN,ed so its anyone,s guess .
Thanks a lot guys been a great help
Paul