Quick update on this - I got a couple of old BT dongles from back in the day when RPi's didn't have BT on them and tried those.
My Win11 machine did not like that at all - Plugging in the dongles basically took the Intel BT drivers down and then rebooting the machine let the Intel drivers take down the Broadcom ones - Bit of a fight back and forth...
Anyway, I thought well maybe I should run one of the BT dongles under a Win10 VM I have - So I powered up VMWare Workstation Pro, loaded in a Win10 VM, and opened the device manager to get ready to plug in the dongle.
I noticed that there was already some virtualized BT hardware there and decided to see if that could see the U1177A.
Yes, it could - It was discoverable there so I tried connecting, and when the Win10 instance popped a connection pin dialog, so did the Win11 host. Typing in the pin connected the U1177A to the Win11 machine and now it works on the Win11 desktop without a problem:
So I think this pretty much tightens the problem to the Win11 BT stack having some problem with the discovery of the U1177A because once connected it works fine.
Anyway, hope this might help someone else who has this issue in the future.
TonyG