I contacted Amazon about this problem and they came back and said they noticed that the country setting in my Account was not set to the correct location.
For some reason it changed after I set Spotify as the default Music app. I couldn't select Spotify in the first instance as it was greyed out, so I changed the Country from UK to USA, and back again - so it must have went out of sync when I was faffing about with these settings.
Here are the instructions to fix the issue which worked for me.
You can change your country settings by visiting the Manage Your Content and Devices page and following the steps given below:
1. Go to
https://www.amazon.com/mycd.
2. On Manage Your Content and Devices, select the "Settings" tab. If you are using a mobile device, select the menu icon, and then select "Settings."
3. Under "Country Settings," select "Change," and add
4. Open the link from the "Click here to learn more about other Amazon sites you are eligible to shop on based on your country of residence."
5. Select "Learn more," and then confirm your new location under the section "Update your default Kindle store to Amazon.co.uk." If prompted, enter your Amazon account credentials.
6. If you have already activated your device, go to the My Devices tab in the Manage Your Content and Devices page, and deregister your device. Register the device again using the Alexa app, and then select your default language.
If you are still having issues, you can reach the chat support representative by using the below link:
https://www.amazon.com/clicktochat-alexachatsupport