It is out of warranty, right?
If so, you have to buy a new one. WPA281 can work with new models. No worries.

I've ordered new WPA 4220kit AV500 as cost of wifi extender cheaper than bought on its own. Does this mean that I will be able to link old & new to new wired adapted so that I can use them in different parts of the house? If so will each wifi extender have to have a different name or would both be part of the same network?

Standard PA211 capacitor failure. They used junk components. If you bought a set of PA211 you can expect the other one(s) to fail soon also. It is repairable if you are capable of replacing the capacitor.
But don't even think about doing it if you are not competent.

http://www.kphonline.co.uk/2012/12/repairing-a-dead-tp-link-pa211-powerline-ethernet-adapter-part1/

There was nothing to lose by looking inside old PA211 but, although I found screw under label I still couldn't open it. More to the point I now have new WPA4220 kit. It works fine, much easier to set up than old kit. However so far unable to access wifi adapter on my iMac to change password and rename network although I followed Mac instructions to the letter. Fortunately El Capitan has enabled password to be shared with all Apple devices.If I want to pair my old WPA 281 what is procedure?

Paring guide:
http://www.tp-link.com/en/faq-258.html

The utility for MAC cannot run on El Capitan?
http://www.tp-link.com/en/download/TL-WPA4220.html#Utility
http://www.tp-link.com/en/faq-398.html