Does it have to be brand new? Otherwise I'd suggest looking for an Ampeg V4B, Marshall VBA400 Sound City 120 or 200, or Fender Bassman head. For that sort of money you have quite a lot of choice in old valve heads. If you could stretch to around £900 you could also start considering SVT's, Mesa 400's or even Fender Bassman 300pro's if you're really lucky.
As far as new ones go there is the new Laney one, dunno how much it is though. The Marshall VBA400 can actually be found new for about £740 so is well worth checking out if you're really set on a new amp.