I use the 5 to 7 mm stuff sold for alpines. Or I buy sharp sand from Wickes and riddle it and use the gravel left from that.

I buy bags of alpine grit and use that. It does tend to be fairly large grit to aid drainage.

I use 6mm grit for our alpines and house leeks, in fact, I use 6mm for everything!

thanks to all who replied to my question. I am very grateful to you all