Thanks for your replies. This project got put on hold for a little while but I'm back onto it.
I had a look at the USB plugs. They look quite interesting but not quite suited to my needs. I'll post exact specs and more information about what I'm doing later tonight.
I've got some ATMega8515 chips spare that I was going to use and I'm looking into some relays that can handle a lot of power. I'll also get an rs232 interface going to the MCU for feedback and control to/from a computer.
Any information I collect for this project I'll post here.