Thanks to all who responded - some useful ideas there.

Unfortunately I tried and failed to repair the leak. The split was
bigger than I thought and because the bottom of the butt isn't flat
where the split is (it has some indentations, presumably to strengthen
it) it was difficult to stick tape on it, etc.

It looks as if I'll have to replace it

Any suggestions as to what I can do with the old one - maybe make it
into a compost bin or something? Seems a shame to dump it.

Reg Q