Thanks Lovejoy for all the help but fingers crossed I think I have sorted it!!!!!!!! Was about to change Gas valve as per book earth wire then pressure sensing pipe.... Followed that up to the brass conecttion in to the combustion chamber and then gently blow and sucked up and down it!!!! then went to doner boiler and did the same ...wow there is a diference in the amount of pressure that could be sucked down the clear pipe .S o changed them over and ,it now fires up with the door fully fitted. The newer one is a slightly different design ????
So did it wear or is the gas valve getting on abit four years and its tolerance for that in put from the pressure sensing pipe is a bit out. I wonder if changing the whole G.V would have did the trick. Fingers crossed now its ok.