If Unicode still makes problems, check (with a hex editor) if the file you generate has a byte order mark

Thanks. My URL doesn't have a BOM and I hadn't considered the possibility that it might need one!

I have changed the code as suggested by kcott and I'll now wait until tomorrow for Google to hit my calendar endpoint and see if works. If not, another day and I'll look into whether a BOM is the issue. Debugging this is taking forever as I have to wait for Google to refresh which is does approximately once per day.