Thanks. First time I tried I got : "Sorry something's gone wrong. There's been a problem while processing your request. We have registered this problem and will be looking into it shortly."
I then tried again and as far as I can tell it has worked.
How long does it take before it becomes effective? I don't remember seeing anything about that on the webpage about that, but can't check because I am presented with a different page now I have asked to opt out.
Pleased to hear you finally got somewhere.
The best answer I can give to this question is from another thread "
I opted out of BT wi-fi yesterday and have been led to believe that I would have to wait as much as several weeks before wi-fi is 'not active' on my hub."
Thanks. The link to Your products was helpful to see that I am invited to join BT Wifi. Presumably that means I have managed to successfully disengage for BT Wifi. My router settings now also say it is not active. That was not the case when I checked before Christmas. Without the link in your post it may have otherwise taken me quite a time to find my way back there because the website is such a nightmare to navigate. So thanks.
To bring things to a conclusion, I have managed to reset and now only see devices connected that I recognise. That was after both resetting the router to factory settings and turning it off and back on again. After the factory reset I was still seeing a couple of devices I did not recognise - allegedly connected by Ethernet. The disappeared after the power off and power on.
Thanks to everyone who contributed to helping me solve my problem, and best wishes for the New Year.