It's called Petseal.
I used it about 8 or 9 years ago on my Landrover petrol tank which was leaking like a seive.
This goo sealed it up & to my knowledge it was still sound after 5 years of off road abuse.
It should cost about £12 for a can.
Search under "Petseal uk" on a search engine or take a peek in any decent classic bike magazine.
Good Stuff!
Andy