Repairing Plastic Tanks- Success!!!
 

Following several attempts and lots of different methods I've finally found one that works for the 23 litre Acerbis fuel tank.
It's not for the faint-hearted but following advice from some hard core Kayaking mates, it seems plastic welding is the way to go.
After a lot of practice on fuel grade plastic and different heat sources I cracked it (or rather un-cracked it) using a blowtorch and some old oil Jerry cans. You need to be carefull not to melt the fuel tank too much as it becomes brittle but enough to ensure a good bond with the patch material.

Good Luck. George