Hi Ben Sparkes,
Bandsteering and Load balancing is enabled by default on all versions on ZD software so that should not be a issue however there are situation when devices which won't get bandsteered or load balanced because they are adamant (Read: client behaviour) which ZD can't do anything about it.
Even if for a second IPads are considered to be adamant devices and when AP hits the limit why is that your laptops and phones not going to next door AP. Whats different? I know IPADs are the ones which should get balanced among AP's.
Are these on 2 AP's same channel?
have you tried to force to connect to SSID from next door to see what happens?
which version of code, ZD model and AP's you got?
One reason i think is that next door ap may be free to load however ridden with Layer 1 issues hence client don't like to go THAT dirty AP because they find nearest one clean and good to work with.
it would be interesting to see how this gets sorted. Please publish screenshot of those two AP's for quick comparison. See below link and you can find this at Zd GUI >>> monitor >>> AP >>> radio info