To be honest, there are quite a few amps around the £400 mark which may suit you.
Of those I've heard (which isn't any of the new models though), for the most part I don't think there's a huge amount of difference overall between any of them. It's mostly a slight presenation difference, some prefer one while others go for the other.
Fair enough, the Sony's can be a bit forward and "in yer face" so to speak - and I think this can let them down a bit for music (not hugely though), but on the other hand it seems to make them sound that bit more dynamic for movies.
Yamaha are always worth a look too, and you shouldn't dimiss Marantz either. Denon?, well they are certainly up there although I've always felt they were a little overpriced compared to their peers - however the AVR1803 looks like it might change that opinion (not heard it though), and the 2802 looks competitive now on the street (I thought it was overpriced when it came out).
At the end of the day, it'll come down to which has the right blend of features, performance and price.
Consider what you'll be using it for (music/movies/both), the type of speakers you'll be matching to it (ie if the receiver is a little "bright", don't pick bright speakers too), what kit you want to connect to it (no use buying an amp which doesn't have all the necessary connectors), what unique features it has that you will actually use (don't be fooled by gimmicky type stuff like DSP modes (unless you like that sort of thing) but some can be useful - things like adjustable sub xover, input naming (Sony's have this AFAIK- very neat especially if you have anyone technically challenged in the house), dual speaker sets etc etc.
Basically anything which matters to you (regardless of whether I or anybody else would pay it a second thought)
I'd do the research and compile a list of those models which fit all your criteria of features, performance and price, and then go and buy the one you think sounds the best!
Models I'd probably look at, at around £400 or less:
Denon AVR1803 (poss the 2802 too)
Marantz SR5300
Yamaha RXV630
Kenwood KRFX9060
Sony STRDB1080
(some of these aren't available yet though - and I have to confess I haven't heard any of them yet (apart from the 2802), so do consider that too)
On paper the Kenwood seems to offer a very complete package for the money - if it sounds good too......... but check it does everything you want first if considering it!
If one of those doesn't suit - perhaps you need to be looking higher up
