Best way is to look in the signal detection menu item (Menu>Settings>System>Signal detection) which shows you the current tuned channel and if you then press OK or right arrow shows you the other tuned channels. Compare the list to those for your favoured transmitter as a number of common problems are due to the box tuning to more than one transmitter. Best source of transmitter information is http://www.digitaluk.co.uk/postcodechecker/main/index/dummy/NA/yes (tick the "I am in the aerial installation trade" box).
Depends where you are - the HD mux started last April albeit low power and actually transmitted from Lichfield - designed to serve B'ham and other areas served by Sutton. It also has a dead spot towards Nottingham to prevent interference with transmitters over that way. I think it might be moved to Sutton soon as things are moved back onto the old (now taller) mast before DSO and full power in September.
I live over towards Leicester and get the HD mux just fine (even from a loft mounted antenna! - Listed building!)