Well Team, new tank fitted and i did use screwfix. Took more time to find my hole saws then it did to fit the new tank. That said, when i removed the old lagging and had a look at the old metal tank, well! Job done in the nick of time, as the first pin hole started to leak as i undid the first connection.
The tank must have been some 30yr old and it looked it.
Once again thanks for your replies.