If you can give me the make and model number of the STB, Clive (Set Top Box!), I'll find out some more for you. Is it a Sky box?
From what you've said, it sounds like the STB will only deliver DPL (Dolby Pro Logic), when what you really want is Dolby Digital 5.1, and preferably DTS too, which it sounds like you will need an additional amp for.
The difference is this: DPL outputs from the centre speaker (the one that would be above the TV), and in stereo from the front speakers, but only outputs from the rear speakers in MONO. With DD5.1 and DTS you get stereo from the rear speakers as well, which makes a hell of a big difference to the overall Home Cinema Experience.
The reason you have only been given a DPL box, is that Terrestrial TV, and (with a minority of possible exceptions) most Satellite channels only broadcast in DPL. You would only get the benefit of DD5.1 or DTS when playing DVDs, but I can't express how much it's worthwhile having!
You might find
THIS GUIDE interesting, and informative.