Hi st10ss, many thanks for replying!
I’ve figured this out now and wanted to post my resolution in case it helps anyone else with the same issue.
It seems either Cubase or the Behringer UMC22 doesn’t like USB hubs. With the UMCs plugged into a USB hub I get all sorts of issues, like this one.
However with UMCs plugged directly into the PC all the problems go away.
For clarity the UMC22 is a simple DI box for microphone and line-in signals and the hub was un-powered.
Regards
Ian