Iirc there was a post on detailing world that said as a guide you can do a reading inside the door openings. Generally these are lightly clearcoated and tend to reflect the paint thickness before the clearcoat is built up ion the body panels.
Cheers for that. I had no dispute at the dealers bodyshop with the Asst Manager and they didn't even measure the paint depth on my car, taking my detailers readings as gospel. Armed with the manufacturers recommended paint/clearcoat depth, and at my request to resolve the issue to my satisfaction, they have agreed to repaint/clearcoat the roof. I'm not making too much of a fuss about it as the service dept and bodyshop fell out over who was to blame when the original problem occurred - no argument in my view - bodyshop did it

It seems clear to me that all the bodyshop did was aggressively polish out the offending mark and levelled the clearcoat substantially in the process. The low readings indicate this. When the job is done, I plan to get my detailer to level the roof in line with the rest of the car and I'll ask him to take readings from the door openings as a benchmark.
Chiers for the tip

Just noticed tonight that 2 of my lovely 18" Mpnza II alloys have the start of corrosion creeping under the lacquer

Spoke to warranty dept tonight and car will be in tomorrow to have them inspected and (hopefully) replaced - no kerbing at all, one wheel has it just inside the outer rim and the other inside one of the circular spokes

Must have a close inspection of the front 2 tomorrow in daylight
Cass