Hmm, things are not as rosy as I first thought, the BBC channels have been degrading badly just after 9pm each evening. That problem started just after the first phase of switchover two weeks ago, then resolved itself after a couple of days. But it's returned with the second phase, and is much worse.
It makes me wonder if they may be tweaking the signal strengths to avoid interference with neighbouring areas. I also can't help wondering if the transmitters may perhaps be set to change power at different times of day, which might explain the phenomenon others have reported of very transient break-ups, at very specific times, as the receiver's gain control mechanisms reacclimatise to the new signal strength. That's just vague a theory of mine, I've found no evidence to support it.
Of interest, digital TV employs very similar technology to ADSL, with Reed-Solomon Forward Error Correction, and an interleaved data stream. As long as errors are being corrected all is well but, as soon as uncorrectable errors arise, the picture will start pixelating. That does mean that many people who are receiving a 'perfect' picture may be unaware that there is actually a significant underlying error rate is still taking place, but getting corrected by reed-solomon. Then, if the error rate gets just a little bit worse, some of them will go uncorrected leading to the sudden deterioration from 'perfect' to 'awful'