Half the house is on 1 fuse box and the other half on another following a large extension.
Want to use Powerline to provide internet on sockets connected into both fuse boxes.
In the garage there is a single socket connected into the old fuse box and a double into the new. Do I need to connect both up using powerline to connect both "circuits" or will powerline be able to reach round the whole house without.

Hopefully this makes sense.

Internet does not work from "sockets ". You plug your computer, modem and wireless router. if you use one, into a wall outlet for power. Your computer output then goes into your Cat wiring for house distribution, if using such, or from the modem to the computer. Your ISP cable goes into the modem.

If you are using the powerline for internet, You should be connected, Do you have a phase coupler, or a way to pass from phase to phase, or is that not a problem?
A Transformer acts like a choke and will block those signals(It also may work at a neighbors house on the same transformer?

Check:

Linksys Powerline AV 1-Port Network Adapter Kit PLEK400 B&H

And

http://www.howstuffworks.com/power-network.htm