pressure in 90 fuel tank, parts suppliers open on saturday.

hi all, i have a 90 with 19j engine, filled the tank and noticed when i take cap off the fuel tank it has a lot of pressure come out and in the 3-4 miles to work the fuel tank filller cap leaks. Done a bit of research which points to afc diaphram. I am looking too see if anyone knows anywhere open on Saturdays in Essex which might stock the diaphram. I am meant to be at pay and play on Sunday but am not going to if i cant fix or would the landy still run fine all day just lose a bit of diesels. Any advise would be good. Cheers