TP-Link support suggested that I post the following experiences of compatability issues between differing TL-WPA4220 powerline extender versions - as it may prove helpful to anyone with the same problem.

I was hoping to extend my current TP-Link powerline network, based on devices TL-PA4010 ver: 3.0 and and TL-WPA4220(UK) Ver: 5.0, using TL-WPA4220(UK) Ver: 1.0 units: which also are listed as AV2 compatible. 

Unfortunately, pairing and SSID cloning the TL-WPA4220(UK) Ver: 1.0 units onto the powerline completely disrupted the network and made it inaccessible through the TP Utility software.

To try to resolve this problem, I updated the firmware on the Ver:1.0 units - but this did not cure the problem. 

On experimentation, I discovered that it was not pairing that caused the problem, it was cloning the SSID that was the source of the issue.

As an alternative approach, I have also tried cloning the SSID first through an ethernet cable connected to the router. This works, in the sense that the Ver:1.0 unit becomes a wireless access point (wap) with the same SSID as the router. HOWEVER, if you then try to pair the Ver:1.0 unit into sytem, the powerline network is again completely disrupted.

To concluded, the Ver:1.0 devices could be used to extend a powerline network BUT only if you don't try to clone the SSID and only use the SSID and wireless password printed on the unit. However, this limitation may be a security issue.

PS. I have now extended the TP-Link powerline network with an old Netgear wireless AV1 adapter - confirming that the problem is one of incompatibility between TL-WPA4220(UK) Ver: 1.0 and TL-WPA4220(UK) Ver: 5.0 systems.