The difference would appear to be in the operating system.  When Ver 2 of the download app came out, it installed fine on my 64 bit Win PC in the office but not on the 32 bit I have sitting next to the telly.
They've done some tweaks since then and by following to the letter the instructions on the iPlayer web sight in the trouble shooting section (which is mainly about ensuring all aspects of the previous versions are un-installed), I now have it working fine on the 32bit gear.  You will however still need an authorised UK user's login details.