Problem diagnosed!
While a couple of the hoses looked dodgy, pushed them back on and they were fine.
Worked out it only happened when I filled it up, no problems after that.
Then, when I filled up last night realised it repeated, and I didn't get that nice sound when you remove your fuel cap and hear the pressure release...
So mucking around this morning I've realised that 2-3 of the bolts that hold the fuel pump assembly top in have become stripped/damaged. They hold the top in, but not strong enough to seal it.
I'll see if I can get some more bolts from somewhere today and hope that fixes it, otherwise I'll get dad to rethread the assembly next weekend.