I had this problem on an old Mini...turned out the battery wasn't holding charge and there was only enough charge to spin the starter motor but not 'throw' it across to engage with the flywheel ring.

Can you jump it with booster cables from another vehicle?

If you can't then I'd agree with Lower on a possible diagnosis.

 

Thanks, but I doubt it. I had the battery go flat a while back and even then it was still managing to turn over the engine, albeit slowly and painfully. I replaced the battery on that occasion, so the one that's in there is pretty new.

 

Mini's had an inertia starter where the start motor spinning threw the pinion out into contact with the ring gear.

Modern pre-engaged starters don't work quite the same way.