I had the same problem I think, on mine the adhesive that holds the petrol tap boss in deteriorated and subsequently caused it to leak. As you have done, I tried everything but because the tank is flexible it soon cracked and started leaking again.
I did source a new one through Malcolm Rathmell Sport but that was about 2 years ago so I don't know if they are still available.
In the end I was told about a substance called SIKAFLEX, Its like a black mastic but much better. Its flexible, sticks like s--- to a blanket and seems to be petrol or Benzine resistant. I sealed the tank probably 2 years ago and its still petrol tight.
Hope this is of some help