Just an update on this one, I have managed to fix the tank.
I used a sharp knive and sliced open part of the handle so that the seam was exposed. I tried the repair glue that came with the boat but this was no good, I have since used superglue and it has stopped the leak.
I tested the tank by inflating it with air and closing the airvent so that the bladder stayed inflated; two days later it was still fully inflated.
Used it at the weekend only half filled it incase the fuel caused the glue to disolve and the leak started again, but so far no leaks!
It might not last for ever but will do as a temporay repair for the time being. My thanks to all who replied with suggestions.