Playing BBC radio stations on HomePod Mini

So... after reading up about this I managed to get "Hey Siri, play BBC Radio 4" working on my HomePod Mini by creating a Siri Shortcut using the BBC Sounds app action, then a Playback action directing playback to the HomePod Mini. I set up separate ones for BBC Radio 1,2,3 etc and all was fine until a few days ago (maybe a HomePod update? It's now running 14.3), and now they don't work - the HomePod just says "Sorry, I can't play that radio station" etc even if I say the Siri Shortcut name exactly.

If I activate Siri on my iPhone and say "Play BBC Radio 4" then the Shortcut runs fine...

So it seems as though HomePod Siri is trying to interpret what I'm saying rather than recognising that I have a Siri Shortcut which I'm expecting to be run. Does anyone have a solution to this issue?

Thanks, Tim