SPDIF is optical so you shouldn’t need a converter. 
 

Have you removed the end-caps from the cable? 
 

Have you set the TV’s audio output to optical?

Hi  Thankyou.

Yes, I’ve removed end caps. 
 

Bizarrely, there just seems to be no part of the TV menu where I’m able to select digital output. 

The only option is to switch between ‘PCM’ and ‘Compressed’ on the digital output part of the sound menu. It’s really weird there is no ‘select audio output-type option. 

Also, the connector into the back of the TV seems really wobbly. It’s a ‘mini toslink’, which I thought was the right one but it’s really loose!

SONOS uses the connector on the left. Which model TV are you using?

What you probably need is a cable like this one to make it work. The mini toslink will not work in a regular toslink connection.

Thanks have attached a photo of the port below. 
(So the standard optical end wont fit, it is quite an old TV). 
 

UPDATE have just noticed when taking the photo it is marked as Coax. So even if I can solve the connector type, I’m assuming I’ll need a converter box too? 
 

UPDATE. sorry forgot to add the photo!

looks like it’s a coax digital out. 
 

I think it’s not going to work with that tv. 
 

From Sonos’ Ray overview:

Works with your TV

Connect Ray to virtually any TV, and use your existing remote to control the sound. For a next-level gaming setup, connect your PC.

TV or PC must have optical output. 

Maybe this converter will be productive.

Thanks for the replies. 
I am going to have a go at using a digital coax to optical converter. 
Having looked (and tried a standard rca plug for size) I have worked out I was using the wrong connector. But will need to use a digital coax cable to a converter to get to a standard optical cable. Fingers crossed.