I'm trying to add a channel to my favourites in non-freesat mode but I can't find an option to do it. I already have some favourites in the list but what's puzzling me is how they got there. My memory's not what it was but I'm fairly sure I added them previously (they are the programs that I normally watch in non-freesat mode).

Can anyone enlighten me ?

You cannot set up a list of favourite channels in non-freesat mode. The list that appears as 'favourites' is in fact a list of your most-viewed channels in non-freesat mode - see page GB32 of the manual. It is confusing - they should have retitled the tab.
The best thing to do is to sort your list of non-freesat channels so that your favourites are close together at the top of the list or grouped into categories (e.g. general/movies/news/music/other) so that you can quickly move between them.

Thanks