Folks -
I've seen a couple of people stumble with out-of-date mux information (not tvheadend's fault, it ships with old files from linuxtv.org which haven't caught up with digital switchover) - basically, you set your location and promptly fail to find any channels.
Now, I'm sure that you can go back to linuxtv.org and try to update the files... and I know that you can create your own local versions. However, I thought I'd share that the simplest way I've found is simply to add them manually. If you know your local transmitter (which you'd have to to add your location...), ukfree.tv does a good job of listing the mux frequencies along with everything you need to add them. Since you're only looking at half-a-dozen muxes (muxen? muxi?) it's a two-minute job to disable the "bundled" ones and add the correct ones by hand.
As an example, my local transmitter is Ridge Hill; http://www.ukfree.tv/txdetail.php?a=SO630333 lists the muxes as 530.0MHz (so 530000 when you add it), 506.0MHz (506000) and so on. Bandwidth for the SD muxes is 8k, everything else can be set to Auto, although much of the information is listed if you want to use it. Add them in and wait for them to scan and populate...
The only caveat is HD muxes - I don't have a DVB-T2 adapter so can't get these, but I'm not sure of the 8k bandwidth on these and can't test. I'm sure someone will post here eventually if they can answer that one, though.