"Mendip COM multiplexes will temporarily operate on channels 62+ for SDN, 56+ for Arqiva A, and 59+ for Arqiva B (all at 10kW ERP) until Autumn 2011. Final DSO channels will then be adopted, though ERP levels will remain limited until early 2012. "
This means that Mendip has, rather uniquely got these multiplexes:
PSB1, PSB2, PSB3 (Freeview HD), A, C and D
The BT channels are on COM6 in switched areas, and B in non-switched.
Tim - My response won't directly help you on this one, but for you info, I have a Humax Foxsat HDR with CAM slot and it's certainly active on this receiver. Currently I have an AL-Jazeera Sports card in a VIACCESS CAM which allows the motorised capability of the receiver to take from my dish when on 13degrees South East with European Champions League, SPL, etc. I assume therefore that the CAM Slot an your DigitalTerrestrial box will also be active. You can check this out if you can borrow a CAM