Thanks Gareth. Whilst at the tyre shop I took photos of the other two SportMaxx RT tyres on the car, both of which had either 2016 or 2017 dates. The fourth tyre was a generic budget one.
A tread photo of each other tyre (still inflated) are attached. As you can see, they are all cracking after 2.5 to 3.5 years of use. Not sure what kind of mileage or temperatures, but the tyre fitter thought it was not good. Anyway, they're all heading for the disposal/recycling now.
Cheers
Tom