Filled up the tank yesterday right to the top around 6pm and then
parked the car later that evening indoors about 11pm. Today at 2pm I
had a puddle of gas about 1m in diameter under the gas tank by the
driver's side rear wheel. (I was parked on a level surface).

I've never experienced a gas leak before, but have a feeling its from
the tank. Could it be that I just overfilled the tank right up into the
neck and that a seal there is starting to go? Should I replace the
whole tank? Any other ideas what could be causing the leak?

Thanks in advance,
Theo