why just not fit a rcd waterproof socket to outside wall,?and no,a flexi cable not good for outside socket,
if there is a socket on the wall facing your garden,then just turn power off,remove the face plate,carefully drill through the back box,knock out one of the blanking pieces at rear of back box,run a bit of 2.5 cable to the new socket,take it into the socket back box from underneath fit a gland,there will be a blank plate under the back box,it will be round to fit the gland,wire up the socket,dont forget earth wire,put the yellow and green sleeve on it,now screw the face plate on the outside back box,(4screws)
then connect the cables into the face plate in the socket in you home,red to red,or brown to brown,black to black,or blue to bule,depending on how old your wiring is,your new cable will be brown and blue,brown live,blue neutral ,
mm, March 2012