Ok, great - thanks for the replies. Some of my neighbours have slate aggregates in their gardens and I was just wondering whether to ask someone for a few handfuls (no, I wasn't going to sneak some at night tut tut!) so that I can construct my own slate caves, using aquarium silicone gel, rather than paying hefty £20 or whatever it is they charge in shops for a ready-made one.
Ages ago I bought a single piece of "aquarium slate" (triangular sort of shape, about 4 or 5 inches length and it cost about £7!) I thought there must be something special about the slate they sell in aquarium shops to be that expensive. But you live and learn eh? I don't like being ripped-off LOL.
Thanks, Athena