@Hal9000 this is great, but I think I found where it went wrong, and I found an easier way to get the right URI.
In the Sonos app, play the track/playlist/radio station that you want (I wanted to set a TuneIn radio station).
Go to the IDE -> Devices and go into you device. Scroll down and you will get the right string to use - ‘trackUri’ is the string that needs copying ( you mentioned the string after CURRENTLY directly below ‘AVT Play Log’).
This string is what needs pasting into the CoRE ‘Play track’ piston. In the pic below, the line called ‘uri’ is the string you mentioned above. When I used that (I created a test piston that changed the station when a test routine was pressed) the first time I pressed the routine the radio station changed to CapitalFM from Kiss FM but it only lasted 10 seconds then switched back to the Kiss FM. Pressing routine again didn’t switch to Capital FM.
