This post is an update to my previous posting.

Product Model: TL-PA4010P KIT(UK)
Hardware Version: ver 2.1
Firmware: 1.3.0.2134-00_401192_160311_906

Since the firmware update the network works well. However, every 2~3 months connectivity is lost - typically being noticed by either the Xbox One not being able to connect to the Internet or iPads being unable to locate an Airprint printer.

Using Powerline adaptors in a secure network configuration (using my private network name), both devices/network segments are star-connected to my ISP router:

. . . . . . . . . . . . . . . . . |+PA2---Xbox
.Internet--RTR--PA1+|+PA3---Ptr & Office LAN
. . . . . . . . . | . . . . . . . |+PA4---'Hotdesk'
. . . . . . . . WiFi

An investigation using the tpPLC confirms that PA1 has dropped out of the secure network and does not see any other members of that network. The other three PA's, who typically (but not always) remain members of the same secure network and thus whilst they no longer see PA1 they do continue to see each other.

To restore service, I have found it is necessary to use tpPLC on PA1 and save my network name to the adaptor. Having re-established my secure network, I then save the secure network name to all adaptors in my network; this seems to avoid the other adaptors from dropping out, of which PA2 is typically the one to follow PA1.
Following my fix, I have tried swapping adaptors over and have confirmed the first to fail will be whichever adaptor is installed as PA1, followed by PA2.

I therefore conclude this specific product model has a memory leak/buffer overrun or similar fault that causes the NVR holding secure network configuration data to be corrupted - as a simple power off/on doesn't enable the network to be re-established. However, in saying that I note that tpPLC gives no indication that user supplied configuration data (eg. device name, traffic priority etc.) has been corrupted.

Not sure of the solution to this as the latest firmware on the TP-Link(UK) website: TL-PA4010P(UK)_V2_160316 published 2016-03-16 installs the version I am currently running.
lFrom some of the other comments it would seem the ony solution will be to replace with products from other vendors...