Although it pains me to admit it - at the price bracket you're discussing you cant really go wrong with one of the "big three" brands, Trek, Specialized or Giant.
Rigpig makes a good point regards suspension, cheap, heavy suspension is worse than none - if you can find a fully rigid bike then its likely to have much better componentry for the money and much better future upgrade potential if you find you get into it. Another thing to avoid is "rear mech fever" - its an old trick to put a higher spec rear derailleur on a bike to make it look better equipped than the competition - shifters and quality of wheels is a far better thing to concentrate on.
As a first "real" MTB, at that price I'd point you toward the Specialized Hardrock Sport Disc - in my experience the kit spec and the overall look, feel and handling of the Specialized bikes just tends to tip the balance in their favour.
Go for a hard tail , front disc hyd brake . Should be able to get a decent trek/ Specialised for that sort of cash. Just ask yourself. Will you be better at biking than your bike? If not Trek it is mate. Pm me for more info if you want
Don't worry too much about the frame. They rarely break and the dearer ones you are really just saving weight and getting a little more comfort and obviously better construction. What is important is the groupset, forks and wheels. The groupset combines the brakes, gearing and cranks. The least really is Shimano Deore. Anything less then that is prettyl poo to be honest. The forks on a Â£400 bike won't be much cop and will probably weigh a ton but if you're not racing which you wouldn't be on a Â£400 bike does it really matter? Wheels are the best upgrade you can make on any bike though the wheels on the Spesh Hardrock are adequate for daily limited use. Thrash them though and they'll soon fall apart on you.
These are 2 quality hardtails from Kona (Blast and Fire mountain Â£420 and Â£320 respectively). Rutland Cycling have an E-bay site with top bargains. My wife runs the site on their behalf so i guess i'm a bit biased however...It is quality gear and cheaper than most.
They would certaily fit the bill and be good enough for trail riding if you get the urge. I'm a mountain biker too so I have an idea what I'm talking about. Hope that helps.
have you considered a 'town bike' where you can use it on the roads and light trails.
other than that for Â£400 you can pick up on ebay a very reasonable Marin, Specialized or Orange bike if you dont mind second hand.
there is a brilliant web site, i think its whitstanley cycles (will post the correct link when i can find it) where you can buy current and previous year bikes. the previous year bikes are generally upto 50% off for the top brands.