Maybe the link below will help you although I am not very sure what your problem is:

http://forum.tp-link.com/showthread.php?75985-How-to-build-a-secured-powerline-network&region=china

@Vincent,

Thank you for the link, it was helpful.

I had decided for another reset and start from square one and after setting up the three units I found only one network.

I have no idea how I managed to add the private network but I am glad that I have lost it.

One last question for those knowledgeable people out there;

If I remove one of the AV500 wifi units from the plug socket (not the one attached to the router) will I lose the settings and have to reset the unit or does it retain the set up details in a memory?

Cheers