It seemed that it was only the Firestick connection failing; the TV is connected via Ethernet, and my Laptop on wireless was not affected at all. SO The Firestick WiFi receiver was likely the cause. They do seem to fail on older devices (not just Firesticks, I have a few dongles!)

Then I saw a link on Facebook to MakeUseOf, a forum I often look at "I gave my Fire TV Stick Ethernet without Amazon's adapter and streaming finally stayed smooth"

Now, I had already tried this with no sucess a week or so ago and wondered why I couldn't get my USB 3.0 Ethernet adaptor to work on the Firestick when it had on 2 PCs. https://www.makeuseof.com/i-gave-my-fire-tv-stick-ethernet-without-amazon-adapter-streaming-finally-stayed-smooth/?link_source=ta_first_comment&taid=6ad106ad9b6f210001527352

So I had a quick read of the article, and I found the line "Initially, the Fire TV might not recognize the connected Ethernet adapter. This happened to me, which was quite confusing, as the device seemed to be connecting to the internet like normal."

This was followed by: "Turns out, there's a very easy fix. You essentially need to forget your existing Wi-Fi SSID on your Fire TV Stick. Having the Wi-Fi SSID saved causes a conflict between the wired Ethernet connection and wireless. By default, the Fire TV Stick always tries to connect to the Wi-Fi SSID. However, removing the SSID on the device resolves this issue.

Once that's done, you're all set to use your brand new wired connection on your Fire TV. Based on my limited testing, the wired connection (unsurprisingly) is way more stable than connecting my old Fire TV Stick using Wi-Fi."

So, I did all that, and it wouldn't recognise the USB Ethernet adaptor.  Of course, I was getting frustrated as it was dinner time Friday evening and I had viewing plans. 

To make it worse, the Firestick wouldn't power up now. So I applied some logic, bypassed the OTG adaptor, and passed power directly to the Firestick. It came alive! So,  I then put the OTG power connectors inline without the Ethernet adaptor. It came alive! Then I plugged in the USB Ethernet adaptor. After a few seconds, we had connectivity!