Hello again Max.
You could try some Bilt Hamber Deox-c. I bought some on recommendation, but haven't tried it yet.
https://www.ebay.co.uk/itm/Bilt-Hamb...4383.l4275.c10
Alternatively you could try filling it with White vinegar, (probably watered down a bit, but I don't know the recommended solution). Leave it in for about a week, followed by a rinse out with clutch and brake cleaner which will chase out any remaining water.
Failing that there is a tank repair specialist in Reepham that I could put you in touch with. He knows how to clean them inside.
There's no guarantee that another tank will be any better especially if you buy it blind on ebay or whatever.
Where have you moved too? Still in the Swaffham area? The photo looks vaguely like Kimberly?