P.S. I've seen the RHS website reckons they get to 8-12 metres high in 20-50 years. My best guess is ours is 20 years old (that seems to be when everything in the garden was put in), and it's only about 10 ft high and showing no obvious signs of upward growth (tends to grow out more than up for us): so either they vary or you can limit its growth.

Gardeners World says its maximum height/width is 3m. Mail order tree companies seem to say between 3-6m tall, but that it can easily be restricted to lower heights by pruning.