best method for outhouse electric supply?

Use armoured cable. The size is dependent on what you plan to run in the outhouse, but I would use 10mm. One end connects to a 40A MCB inside the house. At the outhouse end it should go to another smaller consumer unit with 6A & 32A MCBs for lighting and sockets respectively. The outhouse consumer unit should also include a 30mA RCD.
Because the cable is armoured it can be routed almost anywhere and can be buried if you wish.

Colin
January 2008