Cool! Thanks @Colin. I've just checked the GET http request out and it's very simple. Since I'm using Ubuntu on an RPi4, curl was already available. MQTT would have involved a 4Mb download of which I'd only be using a tiny part, so that doesn't make sense.
This approach almost solves the first half of my problem, facilitating detection that the Spotifyd Spotify Connect device has started playing. I think I've also found the solution to the second half too - just need to piece it together and it'll be sorted.
Thanks again
