Hey @GeordieShores ,
so sorry to hear that you feel let down here. It seems that to use the in-built feature the TV needs to be on. However, you may be able to connect this to an Alexa speaker by downloading the Alexa app on your phone, and enable the SmartThings skill so you can turn this on with voice commands.
@GeordieShores Unfortunately the Technology does not appear to work with Tv switched off. Certainly should be able to connect a speaker by connecting Smart Things and enabling skills though as @TessM mentioned. For anyone interested more information here https://www.samsung.com//support/tv-audio-video/connect-amazon-alexa-with-smartthings/