I want something with disk brakes, lock off, 20in frame. Budget is around £400.
Maybe its better now, but I am of the opinion, that if anything ruins forks its lockout.
those in the know take it off thousand £ forks (unless they need it) because it hinders the action.
on a fork of your price range it will probably (in my personal experience) be the first thing to pack up.
Also i would suggest, unless your HUGELY tall, getting an 18″
im 6ft 5 and ride a 20″ XC bike, but have no problems riding an 18″ hardtail, and find it much easier to manhandle and chuck around.
lastly, and predicatbly.
have a good look at a Giant XtC or Talon
very very good value for money, they make many parts themselves so pass the savings onto better bits they dont make drive chains for instance.