Thanks to those kind helpers.

Research has revealed the existence of Sodium Bentonite powder and
"BentoMat" membrane, which is Bentonite sandwiched between a geosynthetic
layer.

Does anyone have any experience of these products?

Haven't found a UK price yet but the stuff reads like a real cure-all

Blocks all leaks, natural product, self-sealing, natural-looking

Many thanks again

Nick Walker