Has the cover been tarmac'd or concreted to the pave ment? Lightly tap a hammer around the edges of the cover to vibrate away any material which maybe holding it down. Keys tend to turn anti clock wise to open. you may need to prise the cover gently as well with a scewdriver.
Does it turn off just your property? Check with your neighbours, make sure they are not using Boilers, washing machines etc. let them know you are turning off their water supply or they may sue you for damaged goods.
A plumber will generally only turn off the tap in the street to change a faulty stop tap in the house. Its better to ease off or fix the inside stop tap then to mess with out side. If it is within your propert boundary (generally) it feeds only your property but definitley (not always)
plumbo, December 2010
