I fully endorsed "Dawn" answer, - purchase an appropriate "Key " to fit your main tap (somewhere in the Garden - if can't find -ask water co/ or your neighbour might know ?
If leaking when fully on - leak might be coming up from spindle.
(the straight u can see below the handle u use to turn off)
If this is the case u could try give the "gland nut" a slight turn clockwise (the gland nut is the 1st below handle which screwed down around spindle) Try a slight turn first - then test, - if better try another slight turn - u will need the correct spanner for the task - and don't overtighten !!
(Still go out and buy a "Key" & know where your o/side tap is located) Best of luck
Howard
October 2005