This started happening, briefly, with the new UPC TV boxes too as UPC experiment with trying (and failing) to get any kind of surround sound (Dolby Digital) working properly. It was quickly fixed their end.
The UPC TV boxes though have a separate setting for NAR.
If I remember correctly, the Horizon boxes don't. I used to get NAR on Horizon until I learnt that it was related to the Dolby Digital setting and the way some HD channels (e.g. BBC1) send their audio streams. "Original Language" would default to NAR but "English" would normally be ok.
I'd experiment a bit with turning Dolby Digital on/off and then do some experiments to see if it happens only on HD channels or on all of them (as ony HD channes have a Dolby Digital option). Then post back here and hope someone from UPC sees it and can fix it.