Thanks Victor, that's useful.
But I'm still not quite clear - does the fact that 802.11r utilises 11k&v mean that I would need to specifically enable those on the controller?
I didn't realise 802.11r was so badly supported. I found a list of Apple devices/OS's that (allegedly) do support 11r, though it sounds like anything older than 5 or 6 years might have issues:
https://support.apple.com/en-gb/HT202628
I'm also not clear on the behaviour - presumably the fact that a client doesn't support 802.11r shouldn't mean it can't join the network? These devices should join but just not be able to take advantage of fast roaming, no?
I have done some reading around but I haven't been able to find a document that clearly lays this information out.