OK, that's good as it gives some weight to my thought that switching off the WiFi transceiver improving things.
The next step would be to switch off the WiFi on all Minis you don't require to act as extenders. Reset the network settings, configure for ethernet connection. Then enter the installer menu and switch off the WiFi in there.
I think Sky need to clarify what they mean when they state you can only "watch concurrently on 2 Minis". They do say "connect up to 4", so I'd assume that means the other 2 need to be in standby?!
Paul