Ade wrote:
My allotment has a severe drainage problem so I'm building raised beds,
which I hope will help. As well as manure and compost, I want to add
something else to improve drainage. It's clay soil. What should I use --
pea gravel or sand (builders sand?) or something else?

Topsoil.

http://www.onlineturf.com/partners/onlinesoil/index.asp

Maybe cheaper from builders merchants or similar locally
to you. Cheaper per unit in quantity. Everything depends
on what you (or other allotment holders if they're at all
interested) want to spend.