And so far so good with my strategy. I managed to sieve out about 70% of the substrate and replace it with oat bran plus some Bentonite clay - my theory was this could help with odour control. I put it all back in the fridge, still in a fairly smelly state at that point.
This morning it's daisies in there - well, not quite, but I could stick my face right into the bowl and sniff with no ill effects at all
. Quite odourless. It has worked very well so far.