Fiesta 2015 with Ford (not Sony) radio/cd player built in satnav.

Iphone SE (for anyone who does not know this is roughly Iphone 6s technology in a Iphone 5s casing (which is smaller than 6s)).

Phone connects to the radio and I can make and receive calls fine.

I can't play music from phone through the car stereo.

If I am playing music on the phone through the phone speakers and get in the car and switch the radio on, the speakers on the phone switch off, but on the phone screen it still shows the music as playing.

But I can't work out how to get the music out of the car speakers.

It's probably something easy that someone of my age (55)  can't work out.

Can anyone assist please.

IIRC, iPhones cannot play music via bluetooth to ford Sync radio systems. The only way to play music from your iPhone through the car stereo is via USB. 
I have a 2017 last model ST-Line with Sync 1.1 radio system and it only plays music via bluetooth from an Android phone (Sony, Samsung etc). It will connect to iPhone via bluetooth for calls only. 

My iPhones (6, 6S, X) and ipads have worked fine in my 2014 non-Sony system. Have you tried resetting the Sync by removing the fuse? 

Stupid question but are you cycling the Aux button all the way through to Bluetooth music? There’s 3 functions through that button - AUX, USB and Bluetooth

Thanks Thaifiesta, now working, I did not realise I had to press the Aux buttom more than once to cycle through the 3 functions. I was sure it was something easy.