Hey mate,
As you say, lots out there for £400 budget and you should be able to get a nice one.
AS before, one of the big things to look for is the cache (this is part of the spec of your processor) and it doesnt always follow that the faster the processor the more the cache.
Cache will have a tremendous effect on speed (and possibly stability?) and useability of your new laptop so get as much as you can afford - its different than memory or hard drive space and things like that, so always ask to see the FULL spec.
Dudders laptop is a great little number and the HP are well made; however, jsut having a quick run over the PC World website for you, and heres what I think is the pick of the bunch:
http://www.pcworld.co.uk/gbuk/hp-pav...41431-pdt.html
You are well under budget (so you can buy more bits for your buggy

) but you have an excellent (really excellent) processor speed, with 2Mb cache, a whopping 6Gb of memory, big hard drive and its 64bit Windows so nice and fast operating system too.
Hope it helps chum.