Thanks Mark
I've been doing some research and have sourced some HDPE. I'm going to have a go plugging the gap and welding it together.
Replacing the tank looks like a long expensive job. I hope to avoid it if I can.
On Closer inspection, the tank reads >HD-PE< meaning it is made of High-Density Polyethylene. I phoned Bostik who make an epoxy putty and they say they don't have anything to work with that type of plastic.
The search continues - I might have to look at plastic welding
I’ve heard of folk using epoxy putty for fuel tanks. Apparently it can’t be used on polythene or polypropylene, so I wanted to check the tank wasn’t made of those.