Last autumn started last march for me, but I really think you are from the northern hemisphere and are suggesting it is far more recent than that.

The only provider I have heard if reducing the number of connections working for is Yahoo. Which now encompasses AOL, Verizon and ATT. But I have only heard about actual Yahoo customers and ATT customers having the change work.

Have you tried disabling your antivirus and leaving only windows defender to protect you? Often connection issues arise because of antivirus scanning making things run slowly so connection is not possible, they time out.

I used to have odd connection issues, and found that setting "Enable DNS over HTTPS" and accepting the default provider fixed the issue. In preferences search for DNS and click on the settings button it shows, then the option at the bottom of the pop up dialogue.

I looked up this "John Lewis" on Google and it is not clear how the non fibre connections are provided? Is it wired or wireless? Wireless, while they don't talk about it much can be affected by weather, just not as badly as satellite. So that "might" be a consideration, as can time of day. When sellers oversell their capacity, problems with speed generally surface in the early evening, from the time school comes out usually until people start going to bed. These capacity issues can also be quite local. If the seller only has a relatively small bandwidth from your local exchange and everyone locally has switched because of some "deal" the capacity locally can be exceeded in busy time. Without the problem really leaving your local exchange area. I use the exchange area like phones as they offer a phone service I assume it is based around the telecoms system, but depending on the technologies used it might be a bus at the end of your street. It is here in Australia at least where "hybrid" connections exist..

Thank you for your full and considered response Matt. You are correct, last autumn was only three months or so ago! I think the problem became noticeable after my ISP had a major email problem lasting weeks but that may be my imagination.

I have free AVG protection so I could turn off their anti-virus checking for an experimental period. Microsoft Defender Virus protection is turned off.

I have tried "Enable DNS over HTTPS" and it seems to have made little or no difference sadly.

Any other suggestions?