Fook me with my old S2 this was one of the worst problems I experienced. I wnet through 3 brand new petrol pipes from Ford. I though't my tank was cracked, it was leaking at the tank end of the hose that comes from the tank to the pump, absoloute nightmare, and it was strange because it happend like overnight for no reason!
Well in the end mine was diagnosed to a shitty aftermarket petrol cap sum nobber had put on the car (I should have removed it anyway) but thanks to Christain for pointing it out that it could be why

Anyway, the standard item alows the tank to breath air in air out (apparently) this one wern't and the pressure was too much for the tank and it kept takin a piss.
And everytime I removed this crap fuel cap the pressure it let off was mega, so though't it may have been that, anyway standard item went on and all was fine. Luckly the tank didn't implode or anything serious.
It did however cost me about 20 quid in optimax and burn/stain my skin and went in my face / eyes which was fuckin not nice I can tell ya!
Oh the joy's of RS ownership
Live n learn