Thanks for replying Barry, really appreciate it.
So what you proposed I do is pretty much what I had done, the only difference being, in the Teams Message field, I was enclosing the Dynamic Content in square brackets as a way to highlight what was the users response, like so (UserOptions is the checkbox field):

Looking at your response made me rethink using the square brackets, and when I removed the square brackets from the Teams message, everything worked.
Looking at the JSON, I can also see that the values themselves are enclosed in square brackets, so (I'm new to JSON too!), somehow enclosing the responses in square brackets in the Teams message appears to "clash" with the square brackets in the JSON/response.
Anyhow, all sorted now though I don't fully understand why it broke - maybe someone might be kind enough to explain further (for a more complete understanding)?
Thanks for your help!