I have been doing some testing by turning everything off and then back on again after a period. There are only two devices that are normally always on - the pc and the NAS.
The activity drops off when they are both power down. I think it's the PC that is the source but the activity doesn't restart immediately, so I am not 100% sure yet. I wasn't careful enough in my testing.
I have also found (since I installed the dsl stats program and can track download volumes) that I have downloaded some 60 Gb in less than a week. That's my normal monthly amount, maybe even more. I suspect malware of some sort and I have left the PC off-line and the virus checker running to see what it finds. I'll report back when I know more.
What on earth is being downloaded, I don't know. (it isn't Windows 10 as that's already installed on the PC). I haven't done the obvious thing and looked for newly changed files yet. (I'm away a lot at the moment and have limited time to pursue this).
Thanks for the suggestions
Edited by filbert42 (Thu 17-Dec-15 19:44:12)