TPK Music handsfree kit - no sound on RCD310 radio

I'm sure I've not understood something obvious here. I recently bought a 2014 Mk5 Polo which came with a TP-music handsfree unit and an RCD310 radio. I've paired the handsfree with my phone and can make calls fine. I can get a list of tracks from my phone on the handsfree and select one. The seconds remaining ticks down, but there's no sound !! (The radio plays FM, DAB etc)

I'd be really grateful if someone can suggest what I'm missing !

Thanks,
G