We're having the same problem. So maybe it is caused by changes that Channel5 have made but surely it's not unreasonable of us to expect NowTV to at least raise this as an issue with Channel 5 on behalf of their customers.
We have a black NOWTV box and been unable to watch Demand 5 for 4 weeks. Chatted with NOWTV's chat lhelpline who suggested resetting the box. This I did but it made no difference. They seemed unaware this problem is affecting 100s of their customers and are doing nothing to sort it. Very poor service.