Fuel leak

[Cloggy Saint]

Hi All, it's been many years since I posted, good to see the place still ticking over.

I have a big fuel leak that seems to be coming from the top right side of the tank but only when the engine is running. I've not had a chance to get underneath and give it a proper inspection, hopefully it's not a rusty tank as it isn't excessively old. What's the most likely culprit? Cheers.