OK, just picked the car up after it's 2nd service (22k and 2 years). It also had a health check, to which I received a video message on my phone (while sat 2 miles away having dinner) by the mechanic doing a walk around under the car. He checks the suspension, brakes and tyres (using a depth gauge) and talks as he does it. To my surprise, the tyres are far better than I expected. After 22k, the original Blue earths are 4mm and 3.8mm (driver side) at the front, with both rears at 5mm. I don't hang around while driving either. I am not a hard driver, but certainly don't 'potter' about.
I thought these figures were really good for the OEM tyres after 22k, and won't hesitate to use them again.
They don't rotate tyres anymore, because in their opinion it doesn't serve any beneficial purpose.