I have now been informed that crushed walnuts are also known as lizard litter. And they are available from pet shops. I will be looking once the girls are back in school. It exists because I have a look online.
If you don't live anywhere near a pet shop, or the idea of it being labelled lizard litter puts you off, then the very helpful Linda sent me a link to this website, where they sell it specifically for pin cushions. Personally, I think I will go for the litter. Makes me smile.
And the last pin cushion I made? It is destined for another friend. She doesn't know it, but she will when it arrives in her post box. I dipped into some precious Heather Ross for this one and love the result.
I hope she likes the result too.