Thank you Jimmi Neutron1
This has been driving me NUTZ for months.
The Gmail calendars create in my work Outlook, creating all the events events during the first sync, but they fail to update - even after waiting days later.
The Onecal.io you recommend seems to work perfectly after a little setting up.
I'm currently on the free trial. But as I am syncing 3 calendars [2 Google to 1 Outlook] - it comes out as an Essential plan which is 100 USD/year. I'm not sure that is worth it though. Damm a bit expensive for a basic function....
UPDATE EDIT
I've just completely deleted the Onecal.io account, it was replicating events and missing others etc. Just not consistent enough to warrant the potential cost. Back to the drawing board.