I wouldn't dig anything in. You will just bring the clay to the surface, and in any case it's very hard work. Anyway in the Summer you won't be able to dig clay, unless it's a very wet season, it will be hard and cracked. Whatever you use, just lay it on top. The worms will take it down anyway. Sharp sand (only sharp sand, not builders sand) can improve the drainage, but I personally wouldn't use a lot of it. You need organic matter. The riding stables on the South Circular (just past the old Harvester) have manure, however you will have to let this rot down before you plant in it, so you may not have the space to do that. Often there are people on here giving away topsoil, however it varies very much in quality. You can buy it in bulk, but that's an expensive way to do it if you need a lot. You can make your own compost (I think the council are selling dalek compost bins cheap at the moment) however this takes a long time and you might not want a dalek in your garden unless you can hide it somewhere. Leaf mould is also a good soil conditioner, but that takes even longer to rot down. I just googled and found this article, which looks useful: https://www.rhs.org.uk/advice/profile?PID=620 Good luck! The only plus point about clay is that it's very fertile, but otherwise it's a real pain! ETA: Sorry, I see much of what I've said has already been said, duh.