DEFINITIVE ANSWER FROM GARMIN
Further to the issue about limited memory space for updates, I emailed Garmin to ask if they could make available a UK only version of the update, as this would fit the available memory (although at the cost of losing the European mapping)
The reply from Garmin is quoted below
Thank you for your email.
We have checked on our end and the last version available for your device is City Navigator Europe NTU 2023.10. Could you please tell us if you already have them installed?
Due to the fact that is a quite old one and there is a limited internal memory, other updates won't be available.
We are really sorry to give you this information but we cannot do anything more, I'm afraid.
So that looks like the end of the road. My Jazz was first registered October 2018, so I would think that any Jazz prior to that date, and perhaps some younger cars, will have the same problem.
It is really annoying. I would be happy to pay for annual updates, and I would have thought it commercially viable for Garmin to provide them - a captive market of thousands of cars stretching years into the future