Strange fuel problem on 2002 KA

Hi all, this is a bit of a strange one, yesterday I noticed an apparent fuel leak as there was evidence of petrol leaking out of the fuel cap down the side of the car. I took the cap off and petrol literally started spraying out everywhere, so I replaced the cap, thought what on earth is going on here, then undid the cap again slowly and it was ok this time, although there was fuel residue sitting on top of the spring loaded flap into the tank, which I had to press in with a stick to allows the fuel back in.

I drove it around for a couple of miles and checked again and all seemed to be back to normal. All very bizarre. I initially wondered if it was due to me recently parking in a new location which is on a slope, pointing upwards, and also with about three quarters of a tank of fuel. It was also pretty sunny yesterday so wondered if it was a combination of all of that causing the issue, but then did some research on the internet and it sounded like it might also be a blockage in the breather pipe? Although in that case that seemed to point to possible fuel starvation which I have not seen evidence of.

This morning even more odd was the fuel gauge was up and down like a yoyo whenever I accelerated or braked.

Any ideas what is going on here, thanks in advance for any advice!