There will obviously be a difference in build quality and materials used in the manufacture but that's not to say that £400 won't get you a good bass. It depends what you're after in terms of style. Active? Passive? Modern in design? Something more old school? Either way I'd buy a yamaha RBX or BB series or one of the higher end squires, Classic Vibe or Vintage Modified. Ibanez might also be worth taking into consideration if that's your type of thing. I'd also highly recommend buying second hand from our classified section where £400 will buy you any number of fantastic basses.