Zwift has come up in a couple of topics so I was wondering how easy it is to use without a smart trainer. I've got an old cyclops fluid sitting in the garage, presumably if I got some form of ant+ speed sensor I could link it with my garmin watch and go from there?

Yes a "Classic" or "Dumb" trainer will work if you have a speed sensor. This is the official guide but others on YouTube.

https://zwiftinsider.com/getting-starte ... c-trainer/

If anyone is doing this can they PM me asap as I have a writing commission and am keen to understand Zwift with a dumb trainer

A speed sensor is deemed non-essential so back to the drawing board