Thanks everyone for your replies - really useful.
I had a look at the HSE links and as a result decided that I'd look for a lift manufacturer who knew what they were doing!
Xavier's mention of Twin Busch was really helpful and I think I'm going to buy one of their
242A lifts that's been narrowed by about 500 mm (which can be done if you have them install it for you).
Phil - you're right, I hand't noticed the lifting height limit on the original link I sent! I hadn't appreciated that the arms would stop quite a long way short of the tops of the posts.
My garage is in the process of being built at the moment and the lift can't be installed until 28 days after the concrete has been poured so there'll be quite a wait until it's in and working.