I rang Michelin tech support again and:
The bloke confirmed that there will be a slight increase in grip as the tyres wear from new peaking at around half worn.
He confirmed that as you approach the wear bars the tread wear reduces due to the tread not heating up as well.
He said that from about half worn the grip plateaus until the wear bars.
He said that in normal usuage (12k miles) he didn't think heat cycling would play a part in changing grip (making the tyres last say > 50000 miles might well introduce tyre compound hardening through heat cycling).
It seemed to me that increased tread life at near the wear bars should equate to less grip and I pressed him on this a couple of times but he was adament that there would be no significant decrease in grip from the peak at about half worn to the wear bars.
Don't know if this makes it any clearer.