Took M2 back to Mazda. They're recalling all M2s in batches to replace the front coil spring. I believe that's something to do with the shock absorbers, though what the problem actually is, I'm not clear about. Still, it's a free service and they gave M2 a long overdue wash and vacuum too. Job done in less than 3 hours. Didn't feel any different driving home, but will keep monitoring...