Hello jimbob-winch,
Welcome to Apple Support Communities. Keeping your Mac up-to-date on software is super important, and we'd be happy to look into this with you.
Does this happen when you connect your Mac to another wireless or wired network? Also, let’s try to download and install the macOS update from the App Store using this link:
macOS Big Sur
Be sure to disconnect any peripherals from your Mac as well before trying the install again.
If that didn't help, let's try to try again after logging into another user account on your Mac. Here's how to create one:
Set up users, guests, and groups on Mac
We hope this information is useful. Kind regards.