Thanks chaps. I'll try welding a scrap of similar polyethylene using a soldering iron and for belt and braces use some Sikaflex inside (works well underwater on my boat). The split came from resting the butt on a couple of casually built dwarf brick walls. A smooth paving stone would probably...
My Plysu brand HDPE waterbutt has a 30mm crack in the bottom. Nothing much sticks to polyethylene and all my attempts to seal the leak have failed.
Any suggestions?