Thanks for your comments and advice.
I've set up the Quality Monitor, but now need to study the chart and learn what all the elements mean.
Minimum latency is 10ms, but average latency bumping up to 20 for periods, with many spikes of 60-100ms max latency. I wonder if this is the cause of our sporadic dropouts...?
Anyway, I rang Sky to ask if they could see anything. As expected, they tested the line and said it was shopwing a constant 55 Mbps.
After a chat, I came away with three suggestions from them:
* They don't recommend the use of Homeplugs (the guy suggested that the Sky Q installer should have refused to install with Homeplugs, although I have been using them for many years)
In any case, he suggested that they can interfere with the SQY broadband
* My Sky Q mini Box has been turned off for the past week - seems the extension cable had failed; I re-plugged it into a wall socket and it came on. He suggested that the Xbox was previously connecting to the SKY Q mini box, but when that stopped working it was connecting to the main SkyQ Box downstairs. I DID explain that the Xbox was actually connected to a Homeplug in my next to the Xbox, but he seemed to think the XBox would still be using the SKY Q Mini.....
* He suggested that I had a large number of devices connected to the internet and this might be causing the dropouts.
I don't fully understand this issue about 'number of connected devices' - although your suggestion that a large number might cause an issue with SkyQ.
I'm assuming most households are like me and have a large number of devices connected to the home network, though crucially (I thought) weren't all 'using' the internet at the same time!
I have the Sky Q Box, my main desktop, 5 monile phones, 3 Amazon Echos, couple of ipads, 2 or three laptops, an iKettle and perhaps several other internet-enabled devices. But only the Sky Q, desktop and Xbox one are typically in use simulaneously. And surely a 55Mbps connection sjhould be able to cope witht his. The phones, Echos etc, might be used briefly from time to time, and perhaps might be making brief connections from time to time, but I assumed this wouldmn't impact performance.
The alternative would be to disconnect the mobiles and the Echos etc until needed - surely not practical...
Anyway, I'll see if the SKY Q Mini box being on makes any difference.
And will also check the monito periodically