echo dot won't find my Sonos Play 1

[travellingkiwi]

I have 2x Sonos speakers. A Play 5 in my lounge and a Play 1 in my office. Alexa has found the Play 5, but refuses to believe that the Play1 is on the net. Both devices are on the same Wifi (Sonos on 2.4GHz, Echo on 5GHz). Both have an IP. And both are pingable.

My iPhone (Also on the same wifi 5GHz) can stream to the Play 1... Why is the Echo Dot blind to it?