Thanks - it's vermiculite I need then. I will get some next time I'm at the garden centre or Wilkinson's.
In fact I was in Wilkinson's today and can't resist buying the odd thing or two. I bought three gravel trays which were reduced to 40p each, a dibber for planting leeks and also some plant clips which work like pegs all for just over £2.
My garage now looks a bit like a hardware shop what with all the cell trays, empty coke bottles, toilet roll middles, various plastic containers etc...