You're not going to get much of a machine for £400, and very little benefit from building it yourself.
Buy one of these monitors:
http://www.ebuyer.co.uk/UK/product/1048 ... 8552657322
And this (basic) PC:
http://www.ebuyer.co.uk/UK/product/1025 ... 8552685323
And it'll come to a shade over your budget, but at least you can just unpack it and use it. Fine for office applications and internet surfing, but don't expect to get any speed for gaming or encoding.
You need to at least double your budget for a decent machine.