I called customer services who were first class and sent a replacement out the following day.
Attempt 2 with the replacement product and exactly the same thing happened. Removed it and tried the one that I had previously held in reserve and again, just the same. I then looked at the screen on the A3 and could see (with the aid of a straight-edge) that it was actually the Samsung screen that wasn’t / isn’t perfectly flat – and dips away slightly around the buttons.
I carefully removed the cover and applied it to my wife’s phone, and it went on perfectly.
In conclusion, there’s no problem with the product, it’s easy to apply, easy to avoid bubbles and it looks good (on a flat screen phone). However, this type of tempered glass cover isn’t going to pull to the shape of your screen if it isn’t completely flat at the edges whereas I guess, a plastic one will. So the choice is yours.