I was waiting patiently for MS to fix this problem but I was getting older by the day.
Today I printed off all the suggestions in this thread and reinstalled 24H2. Sure enough the WiFi didn't work. So I started working my way down the list of suggestions provided by Arvind Kumar. The very first item did the job.
* Uninstall WiFi Adapter - restart computer and let Win install the default driver.
Got my WiFi back but I'm not sure if this leaves me with the correct driver installed though. The last step says:
• Restart your PC to allow Windows to reinstall the default driver, or manually download the latest driver from your Wi-Fi adapter or laptop manufacturer’s website.
So now I have the "default driver" installed. Don't know if that's different from the one that wasn't working. Or it was the same driver but it had been messed up by 24H2 and just needed replacing. Or something.
Now I'm confident I can get my PC working so I might check out drivers. Maybe go to the manufacturer site and take a look, see if I've got the latest driver.
I must say this is a bit dodgy. I worked in software development for years and this wasn't hard to do. I'm pretty sure my wife wouldn't be able to do it. And she wouldn't have known that you can roll back upgrades, or even realised the problem had been caused by an upgrade. And without internet connection she would have had no way of finding out. I figured it out by using her computer to search for info about the problem but lots of folks only have one computer.
Sorry this post got long.
Anyway - thanks for your help Arvind