First, no sand at all - it isn't a good component. I'd contact the British Cactus and Succulent Society if you get no responses here - I don't know what's available across the pond so I'm hesitant to give you my recipe, but I can assure you sand is not in the mix. You want a porous mix, and too many folks operate under the mistaken impression that sand will do it for you. One thing you're likely to find is poultry grit - it's a major component of my mix. Make sure it's completely granite (as poultry scratch has grains in it).