Kerplunk is correct in saying that modern computer monitors are not suitable for accepting the output from a digibox.
If you happen to spot one of the old-fashioned monitors (the sort with AV plugs for the input) going cheap (i.e. for a few quid) in a second-hand shop, then buy it. I've not got a proper TV; I just connect my digibox to an old Commodore monitor using a Scart-to-AV lead �1.97 from Tesco Extra. (The picture quality is excellent).
If you're not lucky enough to be able to get hold a cheap old-fashioned monitor, you can buy a 14" portable TV, with a Scart socket for the digibox, for just under �50 at either Tesco Extra (+ some larger Tesco branches) or Asda.
Chris