Thanks for all the helpful replies on this. I did as it happen consult DIAGNOSTICS & REPAIRS | SatNav Systems as their website suggests they are the experts in this kind of thing, and they replied (confirming what I think we have already concluded) as follows:
The Traffic/TMC (Traffic Message Channel) stopped working on VAG Group cars as of 28th February 2023 due to the licence not being renewed.
We are not aware of any workarounds to get TMC on the Columbus/RNS-510 any longer so the only way to get live traffic is on your Smartphone using an App such as Google Maps, Waze, Apple Maps
They actually work far better than the TMC used to!
I think I have now drawn a blank.