Hi from what I gather reading your post, it would be fair to say that introduction of TL-WPA281 and Powerline TP-link TL-WPA4220 causes a disruption in the network. Also you have the Virgin media Hub which acts as the base station and Apple Time capsule and TP link devices which act as access points.
As per your post Virgin media hub and Apple time capsule worked fine with each other until you introduced the TP link devices.
Did you introduce there TP link access points in the network at once? If yes, can you link only one of the TP link access point and check if it causes issues with the network. If it causes issues then it is safe to assume that it is something to do with that access point.
If it works ok then you can introduce the second one and check if that causes any issues. This will enable us to pin point exactly which device seems to be causing a problem.
Also please correct me if any of my inferences above are wrong.