Several people have assured me it’s nothing to do with my gardening skills (I was beginning to wonder) but that it’s far easier just to sit the stone on top of a pot of compost. So that’s exactly what we did. And because it was a bit chilly we popped the pot into the heated propagator which happened to be on because we were germinating some seeds.
A week or so later a neat crack appeared all the way round the stone so we suspected something was happening. And a few weeks later we lifted the stone off the compost to have a peek, and hey presto, the start of a healthy root system. It’s now back in the pot of compost and we’re waiting with bated breath for a stem to appear 🙂