"When we scanned for TV channels, we found we were missing a couple: BBC 3 and BBC 4. We tried extending the range of the scan and in the end captured them all, but still haven't worked out why the scanning was inconsistent".

One solution is that BBC 3 and BBC 4 do not give out a strong signal until they broadcast at 7pm. You might therefore have no problems with the scan from 7pm onwards. A bit of a pain, but it seems to work for me.

Daniel.