I've been meaning to change my rear tyres for a while, and finally got around to it today.
Holy MOJ, they were in a state when I had a chance to inspect them properly off the car.
It's fairly well observed that the inherent suspension/alignment set up on 171s can lead to uneven wear on the fronts - the inner edge suffering badly, but this was the case on my rears too.
After around 18 months and 9k miles on Uniroyal Rainsport 3 (255/35/18) not only was the inner 25% down past the markers, but there were huge chunks of compound missing, and splits across many areas of the tread. Also, the walls seemed to have developed weakness. Hand on heart, I've tried to baby this set of tyres, but inevitably the torque of the 55 is hard on them despite the ESP kicking-in early.
The car passed MOT inspection in April with no advisories, so I'll assume our hot summer accelerated the wearing.
I still like UR RS3s and never expected them to be high mileage tyres on a 55, but my takeaway advice is to check the inners on both front and rear more carefully than just a quick glance. Beware though - several 171 owners have reported shredding their fingers on exposed steel braiding on fronts...
The car feels much more composed again now.