plug something into the socket, sensible like a radio or the like
turn it on
go to the fuse panel and start to turn the circuits off
when you have found the right one, make sure it's kept off
then do what has been suggested above and use a blanking plate and a waterproof fitting for out door use and fit it in place of the existing plug

if it were me though, i'd be making sure that the wiring to the plug was chansed etc as i think the likelyhood is that it's going to be spurred off another socket rather than on the main ring