You say that you have moved house, do you need to retune the BT box ? This might explain why you are missing the channel (is it the only one you are missing?)
Film4 should be on channel 15. It's on the same multiplex as channel 4 and channel 5, so if you can see those, you should also see Film4.
If you can't see those, then it could be an aerial problem. Film4 is on a different multiplex to Film4+1, and the reception strength for each multiplex can be different.
When you re tune, do you get the offer of selecting from different regions?
I found that, in that case, the box tuned to the first, lower strength channel, rather than the selected region. I cured this by inserting an attenuator in the line and retuning . This lead to only the main region being offered