In the Hot Rod and car restoration world Fuel tanks are often repaired with a white epoxy type substance which may be of some use. You would have to remove the tank to use it but that is the only safe way to weld it up anyway because you would need to purge all the petrol vapour. Sticking the fuel neck over the exhaust of another running car for an hour or so is the cheapest way to go about it. The heat in the exhaust helps to evaporate all the fuel vapour from all the nooks and crannies.Yeah i know it's a prick of a job. Plastic welding may be an option because i can still access the point fairly easily where the leak is.
before i got a replacement i was considering getting the tank welded, there is a plastics place in SA (city plastics) that said they *should* be able to do it, they wanted to have a look to make sure but i never got that far. but they seemed fairly confident in being able to do it. they didn't seem too concerned about it being a petrol tank either (i made sure several times they were aware of it)
Hang on, you lifted your car with a forklift?