Many thanks for your very helpful responses.
I think I've uncovered the root cause. Going to the Matter Details table reveals that the Matter Status is stuck on "starting." Similar behavior to what I'm experiencing was discussed in this thread, and the OP never posted a follow-up to say they had got it working. I've tried rebooting the hub and it is running the latest platform update (2.4.3.173).
Maybe it will decide to start up overnight? 
I'll try again in the morning and report back. Thanks again!
