For those who haven’t looked on hrrrmm, other MX5 sites, I’ve located the source of my knocking, the brake calipers.
I’ve spent a couple of hours prodding and poking all the suspension bushings, finding, yes some do look to have done 49k, but no none of them have failed and didn’t seem to be the cause of the problem. The ‘worn’ ones look as if they have some of the rubber infill deliberately ‘left out’ possibly to soften the response to acceleration and braking torque to get some rear wheel steer effect, that makes them look as if they have failed, but mine haven’t.
Which then led me to start tugging at the brake caliper, which had quite a lot of movement on the slider pins and could be made to make a clonking noise even by hand. So, to eliminate it from the equation I took a bungee cord and wrapped it around the caliper where the handbrake cable attaches and the nearest radius arm, Drove over every manhole cover down my road, all of which made it clonk previously, and nothing, just a gentle thump from the tyre.
The next hurdle it seems, is the refurb of the worn parts in the calipers. None of the usual suspects for MX5 bits seems to stock parts for the slider mech, I may be forced to go the main dealer route.
I did spend a time wondering why I never picked up a noise during the test drive at the dealer. Couple of things. First, pre-delivery they replaced all the discs and pads, which will cause the slider pin to be at maximum withdrawal from its bore in the caliper carrier, therefore minimum steadying effect. Second, a shall we say, large salesman was sitting beside me in the car during the test, which would compress the suspension more and make the ride less skittish.
Whatever the cause though, it’s less hassle than the other problems it could have been.