Try out the TEST socket before ringing Sky, as a final step.
The connection has been stable from mid-October to early November.
Rang them this morning and Sky are saying there was an issue, presumably at their end, but the particular call centre person was unable to provide details as to what it was and whether or not it is ongoing.
They suggest a hard re-set of the router for now, but I don't think that will help at all.
Now Sky Broadband Unlimited
Unlimited Sky fibre from October 2017