Turns out I bought the 16" wheels & winter tyres from another forum member (185_jools) which was a surprising coincidence.
I was talking to my local dealership today about the tyre pressure label on my car - it only shows info for the 225/45R18's, whereas with other cars I've owned I would have expected to see info for other sizes. He said the label only covers the wheels/tyres that come with my car, and if I want to change size I need to ask for a new label!
When I mentioned I had bought a set of 16's he immediately said they will not fit, although he has now sent an email query to Mazda to check. I want to believe the advice from the forum, but I'm hesitant to go against the manufacturer's recommendation in the event of any issue down the line such as warranty or insurance etc.
I've not yet tried to fit the wheels as it's still a bit mild, even up north.