[yorkie_chris]

Epoxy resins and the like, while strong have failings. They are very brittle and ceramic like in their behaviour, and also only as strong as their adhesion to the base material.

Welding is cheap enough, chances are you take a drained and removed tank to someone handy with gas welding ... it will cost you £10 or so in a quick cash job.

I'd be asking why it's failed, why has a load of water been sat in the tank so long?