Just wanted to add my experience to this thread. I was also suffering from frequent "no signal" messages or the picture breaking up. Retuning seemed to help for a bit then the next day it would all be gone again. I was about to return the box as faulty until I read this.
Our local transmitter is Rosneath and is only 5 miles away. From the secret menu I found out that box was tuning to Darvel for some muxes. We're no where near the intended reception area of Darvel and it has the opposite polarisation to Rosneath but the NowTV box was managing to hang on to it with a signal strength of about 19%.
I've added a group C/D filter to our aerial to block out Darvel and the box is now working perfectly with the expected 100% signal quality from Rosneath. The filter cost less than a tenner so if anyone else experiences this problem that could be a quick and simple fix. Just check which aerial group filter you need for your own local transmitter.
The problem I now have is it refuses to display properly on my 14 year old plasma....
If anybody is interested i have linked an article below about "Good Reception Guide" for Digital TV within the UK which is worth a read with some useful advice if you use an over the air TV aerial.