On a TV which can have the PU flashed from a laptop, the software is available and works fine. I don't know off the top of my head if it's available for older TVs which need a new chip. Since you're talking about an HD box I guess you are talking about a fairly new TV anyway, so an engineers visit from your dealer can update your PUC.
You do of course need the Scart cable connected in addition to the HDMI, to allow the picture and sound to be distributed to link room sets.