“There was a long running thread over on Yachting and Boating World (UK) motorboat forum recently about the same issue in a similar aged 34’ Swift Trawler.
In the end Beneteau came to the party and the repair was done by one of their agents in the South of France.
The tank fail was the welded seams and the tank was replaced by a slightly smaller version so it could go back into “jam fit” space allowed.
A big drama with much faffing about which lost him a whole boating season”