Silly questions as I am sure you have thought about it... Is there any EM device near any of the WAP's, either in your place or next door? E.g. electric motors or microwaves? Have you checked if your channels are overlapping with any other WiFi signals (e.g. next door)? You can install a phone app to see the channels. (Netspot on Android is useful).
not really any overlap. Checked that app you mentioned only picks up 2 neighbours really weak. After a few days now though no drops or hangs. New AP's are not near any EM stuff but neither were old ones. Only problem ive got now is my electric charging point refuses to connect to the network which is odd.
So it's all good with handovers now, great, just not just your charging point, I find some odd things like that could happen with some devices and often it was a case of turn them off, restart the network and when everything is up turn the device on. Also just to check you have not plugged anything into the source router, it will be on a different network than your mesh gear, should stop it from working, generally with this stuff everything wants to go through that to see it.