My home and contents insurance won't cover this and nor will my British Gas Homecare agreement either.
It's either the upper joint where the hot water comes in from the boiler that's gone awry OR the whole tank needs replacing.
So that's £50 or £500.
The leak is so slow that I didn't notice the damage it was doing until I looked up this morning.
Until I have the money, I'm wicking the water away to a bucket using a tea towel.