Saturday afternoon just as the rain came down we had a 30 sec power cut.
So I checked my router stats to see what speed we synced at and I was surprised my line was back to what I used to get before fault at the exchange was fixed a few weeks ago. It was at 6.7k and the SNR was at 5.4db. Kept my eye on it through Sunday and the SNR was gradually going down to I think to at midnight 2.4db. This morning it was back up again at 3.1db. Alas this increase in speed was short lived as it re-synced at 11.30 this morning back to what seem to be my new usual of 6.1k and SNR of 5.9db
My experience of these things is to leave well alone, and don't keep rebooting the router. I just wondered is it possible that during the Power Cut the exchange DLM reset itself temporally?