Solenoid has probably gone mine was like that had to replace mine and was fine.. Pain in the ass to do have to remove rear pillar etc and couple of screws and I think 1 on the inside of the fuel cap can't really remember but I'm sure another member or there will be a guide on votex that can fill you in on what to
can you actually hear any noise when you press the petrol cap release?I stripped mine down and the white wheel that is made from plastic when its used the teeth wear down and tiny particles come off and get stuck in the other teeth (it will make sense when you take the cover off) I just used some wd and a cloth to get rid of the small pieces of plastic and been perfect ever since
here's a how to guide with pictures! http://uk-mkivs.net/forums/t/115408.aspx