Thank you all!
As it happens I also emailed Michelin as an afterthought, and they've just replied (this is for the Trail type): 6.7mm rear and 4.5mm front.
I appreciate it's very rough and ready, but for me that equates to about 1200 miles per mm on the rear, which corresponds to around 6,000 miles total life; which sadly seems in the right ballpark.
My rear's not going to last another tour anyway, that's for sure! Might try a 'regular' PR4 next time...