OK, so today I jacked up each corner of the car one at a time. Each wheel rotates freely (obviously the front ones a bit less due to the transmission) and there was no play when I tried to wiggle them in the vertical direction. There was a very slight rubbing sound on all of them but soulnded normal enough to my untrained ear.
I will have to listen out for the noise again next time I get up some motorway speed and any changes over time. What is the normal life expectancy of wheel bearings? I just worked out that 100,000 miles means they have done about 50 million revolutions - a useless fact I know but sounds quite a lot and so maybe they deserve to be retired off!