Ok, for £400 you won't be able to get a 'bent, which may have been the
fastest option :-)

What you will need is a drop barred road bike, since it's down hill,
the actuall weight of the bike won't matter (weight is only important
when climbing and accellerating), what will matter is how low you can
get and what your tyres are. 700x23's should be fine. although you
could get thinner, just make sure they are at the correct pressure (120
psi or so).

From memory there are a couple of Dawes bikes in the £400 range that
would suit. you may also want to check the Trek and Specialized ranges,
or of course go second hand (CW often has add's in the back fo rgood
bikes costing about £400)

Bryan