At about 00:26 this morning my connection went down for nearly half an hour. When it returned it connected at 79999/20000 (previously 76221/19999) with the same DS SNRM of 3.3 dB, but a considerably increased US SNRM. Very quickly the DS SNRM dropped to 2 dB and the US SNRM dropped a little, but still a lot higher that it was before.
These are typical symptoms of a power cut, but there was no power cut. I guess there was some overnight work being done in the cabinet.