Having made some major improvements in this problem, it seems now to be worse since iOS 10 on my iPhone.
The problem is that roaming between Unifi access points on same SSID (which IPv6 present, which may or may not be a factor) loses connectivity - a lot of the time. It happens even when using Fixed IP config, so is not related to DHCP server itself.
It had got to being rare after latest Unifi update, but since iOS 10 it is most of the time now.
Trying upgrading Unifi AP firmware from 18.104.22.16869 to 22.214.171.12420 now. We'll see.
P.S. It has not helped!