Iain - you're right. Freeview, or digital terrestrial television (DTT) transmit over MULTIPLEXES, name taken due to multiple screen cinema's were known as Multiplexes, (MUXES/MUX) and there are 6. The BBC own 3 and ITV own 3 (IIRC) and channels are split over these. Usually when one programme is bad, all the others on this MUX will also be bad. For example, MUX 3 (I think) is the one that carries all the ITV channels (1, 2, 3, 4, News) along with E4 and More4. But E4+1 and More4+1 are on MUX 5, so if you can't get MUX 3 due to bad weather for example, you might still be able to watch these channels on MUX5...

More about MUX's, bandwidth issues and stuff....

http://www.dttboxes.co.uk/channels.html