Pound for pound, you actually get more for your money with a desktop than with a laptop.
Desktops tend to be made from off-the-shelf components, whereas laptops have to be specially designed and made.
Also laptop monitors are usually smaller, with 15 inch about the standard for laptops whereas desktops are usually 17" or even 19".
For �400 you can get a pretty good general purpose desktop PC. I bought two desktops from Dell recently for relatives and both cost me about �425 pounds.
Like Ethel says, perhaps wait till after Xmas. Windows Vista comes out in January and many shops will be trying to sell off their Windows XP computers for when the Vista machines arrive.
Or you could just wait for Windows Vista anyway.
For that I got good processor, good memory, 80Gb hard disk, and a 17" TFT monitor.