Back at a desktop computer now. Thanks for the tip. The only time I'd used Google Takeout was when I migrated away from Google Play Music, but in that case it generated 100s of gigabytes of data over dozens of zip files so I didn't do much with it in the end. Looks like it is quite useable for Google Calendar though. I've data going back to 2013, which is when I moved from an Evolution (offline) calendar to the (online) Google one, so it is quite possible it could go back longer for people who have been using it longer. And it turns out that my daughter had been to a birthday party at the same building and floor in 2014, but at a different door (flat 3 rather than flat 2).