I have recently been using my housmates Magna and today after filling up then going for a 30 - 40k drive, when I got home there was a very strong smell of fuel and when I got out and walked around there was a huge puddle of fuel. I opened up the fuel door panel and it was leaking from the cap, and then when I unscrewed it a little it started pouring out really heavily and then when I screwed the cap back on it went back to a trickle.

Anyone have any idea what is causing this and how I can fix it?

Cheers

Sounds like the cap is fudged and the fuel pressure is escaping, you'll need to go get a new one, wreckers should have them for SFA or autobarn/repco etc. stock replacements.

sounds like the breather is blocked.

it should not push fuel out the fuel cap.

As above, sounds like the breather is blocked. From what you describe, their is excess pressure in the fuel tank. Excess fuel tank pressure is usually released through a breather. Venting to the atmosphere causes more pollution, so the vent is run through a fume filter. The breather will be plumbed, via tubing underneath the car, to a charcol canister in the engine bay. If the breather is blocked, the system is designed to release pressure through the fuel cap. Which is of course preferable to splitting the tank.

Ken