Pretty sure Valiant boats are PVC or PU tubes. You will need two pack glue, right temperature and above all preparation time to remove existing glue, and provide key surface, etc. You should degrease, etc. I'd pull back any ateas where the join is damaged until you're satisfied there is a solid bond already in place, then work back from there.
PVC Adhesives for inflatable boats, dinghy and RIBs