Just an update, SKY, after doing a 24hr diagnostic test decided to reset one's connection and it is now up to it's previously dizzily fast speed of 1.65Mbps down and 0.52Mbps up again.
It would appear that their diagnostic system had detected instability on the line over a period (cause unknown) and it had limited the line, somehow without informing them. Hopefully that's that for the next while.
SHJ