Louisa, Other than the standard advice to add tons of comosted plant matter my only suggestion is to start a red brick factory and use them to build raised beds to fill up with some decent dirt to plant in! I think I would rather have my rock and gravel "soil" than clay! at least with the rocks I can use raised beds of soil and still have drainage.
How deep is the clay? Does it drain at all? I know from the pics of your garden in England that you will find a way to make it beautiful! Congratulations on getting moved into your new home.
Louisa, you should also looking into what's called "Lasagna gardening". It involves setting up layer after layer of various organic things (like leaves, grass clippings, hay/straw, etc) right on top of your soil.
Then you plant right in the middle of all these layers. Apparantly the yields are very high.
Do some searches here for more info on this. A search for Lasagna should produce all the results you need.
Oh, also, after the end of the year, I'd think you could till the whole mess under and add lots and lots of good stuff into the soil. Eventually you'd have some great garden soil!
Thanks Zanymuse for your kind words and advice. A brick factory???...lol! Yes, I too would rather have poor sandy alkaline soil. Thanks to you too Dave I am going with your advice and throw in as much 'stuff' as I can, including cow and pelletted chicken manure. Meantime, I can dream of the future!!