Mate, the only thing I have found that sticks remotely to xlpe is crazy glue. I fear that depending on the size of the leaking area you may have to purchase a new tank. Dale
I've plugged small leaks with Loctite Master Gasket. It stays a bit flexible, but being anerobic, gotta seal it off. Could apply a patch with some self tappers I suppose, using it on the threads too if its a big one.