Arduino to Arduino over Ethernet

I have developed a device to detect a vehicle using a pair of magnetometers.
I now want to communicate the ouput, vehicle detection, to a receiving device indoors, using the mains (240v AC) as a carrier.
I have a pair of Powerline adapters and I have a pair of Ethernet shields, EthernetW5100, and a switch.
Now I am on the very edge of my understanding. I want to use the simplest system of sending messages,
and I believe that I can do this with the above kit, but I need some guidance ?

I would use the User Datagram Protocol: UDP. Try the example File->Examples->Ethernet->UDPSendReceiveString.

Each UDP receiver has an IP address and a Port number. The sending side just needs to know those values to send messages to the receiving side.