The fuel tank in the dart is knackered along with the sender.
Dean you were right, it was fudged up with fibreglass and leaks like a sieve out the bottom.
Right options:
Buy a brand new tank from the US for $400+sender+filler neck+extras
Buy a smaller Alloy fuel tank and mount in the boot £140
Has anyone got a alloy tank or fuel cell sitting in their garage they dont want?