From what I understand, the more modern your car, the lower octane fuels are quite adequate - so an ND should be fine on the cheap stuff.
Our first MX-5 was my wife’s company car in 2011, and it seemed fine on normal fuel, in fact we didn’t give it a second thought. But when the company car 5 had to be returned at the end of the lease period, we opted for a VW Scirocco as its replacement. However, we soon discovered that we missed topless motoring so much, that we looked around to buy another 5 to have just for fun. Unfortunately, we couldn’t afford a new model, and we settled for our current 2008 2.0L Sport Coupe (PRHT). It absolutely hated cheap petrol - it seemed very lumpy, and was not happy at all, but a tank of Shell V-Power cured everything, and now it is my fuel of choice. It doesn’t matter that it is more expensive, because we don’t do that many miles in it anyway.
So, I would suggest that for older MX-5s, Super is the way to go.
In the end though, you pays you money and takes your choice !