I was just going to suggest that you grabbed the latest firmware from OpenELEC, but the two commands Graham posted will already get those for you.
The reason I was going to suggest updating the firmware to the latest version was that I recently upgraded my RPi2 to Jesse and my tuners stopped tuning. They were detected okay, and dmesg showed the correct firmware being loaded, but they would not tune. I updated both firmwares to the latest version from OpenELEC and I was back in business. For anyone wondering, the working firmware md5sums (for Raspbian jesse at least) are:
8e98d25d6219e235b519a3c47dbfa856 dvb-tuner-si2158-a20-01.fw
b2670d8ae5e3369fc71edbb98cdd8f6e dvb-demod-si2168-b40-01.fw
Paul, once you have downloaded the firmware, and either re-plugged the tuner stick, or just rebooted, if you run the command:
dmesg
you should see something like this in the output:
[ 277.318375] si2168 5-0064: found a 'Silicon Labs Si2168-B40'
[ 277.355261] si2168 5-0064: downloading firmware from file 'dvb-demod-si2168-b40-01.fw'
[ 278.111956] si2168 5-0064: firmware version: 4.0.11
[ 278.128797] si2157 6-0060: found a 'Silicon Labs Si2148-A20'
[ 278.139812] si2157 6-0060: downloading firmware from file 'dvb-tuner-si2158-a20-01.fw'
[ 279.409725] si2157 6-0060: firmware version: 2.1.9
That will confirm that the firmware has loaded, and you should then be able to use the commands from the dvb-tools package to scan for channels, or just enable it in Tvheadend and see if it works.
If you want to check the card manually, then you can run something like (I'm using the dvbv5 tools here, but there are others such as w_scan which work the same):
dvbv5-scan /usr/share/dvb/dvb-t/uk-WinterHill
The tuning file for winter hill that I'm using is from the dtv-scan-tables package, but Tvheadend will probably have installed a set of these too (usually in /usr/share/tvheadend/data/dvb-scan/dvb-t). Whichever you use, you need to make sure you are using an up-to-date version that includes the DVB-T2 settings for HD channels (DELIVERY_SYSTEM = DVBT2). You can always grab the latest versions from linuxtv.org, but Ubuntu or Tvheadend probably has the newest ones already. Oh, and don't forget to change it to your particular transmitter (Winter Hill is mine).
Good luck!
Adam