Hi again Netherfield. Been a bit busy this morning myself. Hmm. We know that the units work, sometimes, and for different appliances or gadgets. It may well just be that the stone walls do not like wifi considering the issue seems to get worse the further from the hub and link. I wonder also whether the whole set up is a bit complicated. The Plusnet Hub 1 can be used(I think) with full fibre connectivity so you are likely starting with good speed etc. You have also spent quite a bit on the BT extenders and TP link. Most situations would not require both of those units, and they will not be designed to be compatable although they could be. I think I may have tried a plug in extender (the ones that use the house wiring) and plugged it into each room to check it would work before getting more. I think I would again try the Captive Portal settings on the TP link, and hope that it then allows them extenders to receive and send the wifi.
1 Connect your computer to the access point’s network.
2 Go to
http://tplinkap.net and log in with the password you set for the access point.
3 Go to Settings > Wireless > Portal.
4 Enable Portal
5 Select the effective Wi-Fi network for portal authentication.
6 Set Authentication Type to Simple Password and set a password for clients. Or select No Authentication if you want to allow clients to access the network without any authentication.
What you are doing is setting up a network at home that is similar to the wifi in pubs and cafes, so make the client password a good one.
This link may help.
https://www.tp-link.com/uk/support/faq/2929/