take a look at this site, loads of bikes to choose from and good for checking/comparing specs:
http://www.evanscycles.com/

£400 should get you a quite reasonable bike.
If you are looking for a deal, then often the older models will be discounted, i.e. save £50-£100 by going for a 2006 model rather than 2007 model.

I'm not expert, but I bought a year 2003 'Trek 4500' back in 2004, and it was reduced from £390 to £300. Its been a really good bike, and I've used it most days during the summer months for getting to/from work and using it regularly at weekends/summer evenings for off-road stuff. Been pleased with it and I'd buy Trek again.

The Trek 4500 is around £400 now, in your budget and its spec seems good to me. Another well known make is Specialised, and you can get their entry level bike 'Hardrock' for around £300 - £400 depending on spec you go for. There next model up is the 'Rock Hopper' but that is around £550.

Go for front suspension, steer clear of full suspension for that price. Go to the local bike shop and ask what they'd recommend.