I only fill and empty the ibc from the top access.
The leak must be really low down as it does entirely empty. The bottom valve was both closed, and had an extra screw-cap fitted closed before I buried the ibc!
I can't dig it up- there's a huge
greenhouse and massive raised beds built on top of it! I'd have to destroy and the rebuild the greenhouse!
Put putting clay in the bottom... genius! That I could completely do. Not sure I can really pack it down as I can only fit my arm through the access hatch- but I can certainly add clay, poke it a lot with a tamping tool and hope it settles in a waterproof manner. I have plenty of clay from when we dug out the greenhouse heat sink!
Other ideas from outside of the forum have been a 'pillow tank'- ie a 500L top-access only flexible water tank, put inside the ibc. But these are rather costly.