Problem solved, one of the powerline adaptors was picking up the phones and tablets.
So either that adapter was connected to neighbours powerline network (and not yours), or (if it had built in wifi) was set to allow anyone and everyone in?
This tech might appear to look to be an easy solution to a (non existent, really) problem, but in essence they are all universally shite, insecure, problematic and never maintained.
I've set MAC address blocking on the extender, it's stopped the two phones and tablets showing on the windows network map.
After checking that, changed my router and extender passwords just to make sure.
The fact is, to show under Windows under Network, your computer is receiving valid Computer Browser broadcasts from those devices. To receive them, they must have a valid, usable path for the broadcasts to get through... ...ie, they *ARE* on the same network. Period.
Blocking MACs etc isn't the answer, there is something more fundamentally wrong. Maybe the hub is allowing unauthenticated/unencrypted connections. Maybe the extenders are. Maybe something is wrong with hub, and its allowing BT Wifi onto your private network, rather than VPN'd off to BT? Maybe the device you are seeing this shit on has a poor driver and is connecting to their network.
I think you need to work out what is going on, and educate the faulty device with a sledgehammer.