Well I changed the Carbon Canister today.. and it didn't make any difference. The one that came out certainly didn't seem blocked either.
So back to square one.
I'm still convinced it's the thing at the end of the filler neck inside the tank causing the issue. It appears to be a spring loaded metal plate that seals the tank.. and it's not that easy to push open and does so with a bit of a click.. I can't see how petrol alone could force it open enough not to cause the petrol to back up like it is.
Not sure what my options are now... perhaps a second hand tank?!
Mark