Was going to post a query, but came up with a solution that I’m posting instead. Don’t know if anyone else can add any explanation on this, but maybe it will be useful for other people.
I have google selected as an online account and everything was working quite happily - the mailbox in evolution was set up fine and I think calendars were too. All automatic.
But then I discovered the calendars not working. I added another google calendar and it let me pick from my list, so it had logged in. But that didn’t work either. The two google calendars had an icon of broken network connection next to them (network was working fine).
I went to ~/.local/share/applications/evolution/calendars and deleted caledars.ics (which had a couple of lines and contained no entries - be wary if you are using your local calendar!!!).