First, clean out the pit, and see what sort of stop tap you have down there.
A large kitchen spoon (pound shop one?) firmly taped to a broom handle can save a lot of struggling if it's a deep pit full of mud.
There are many different forms of stop tap in use up and down the country. Some have a 'T' handle like an ordinary tap, some just have a square shaft, some require a hexagonal 'allen key' and some can be turned with a large screwdriver or tyre lever etc. etc.
Some need winding down several turns to close them, and others only require a 1/4 turn.
Modern stop taps are usually fitted with a foam 'frost plug' covering the meter, and a plastic key, which may be loose in the bottom of the pit. They are usually cleaner, and set shallower than older ones too.
As above, handles for older ones can often be created by borrowing bits from a socket set, or even a piece of square steel tube.
Where to get square tube? Your local recycling centre? It's used in all sorts of furniture - I think a piece I have was salvaged from an old bedstead.
A length of 2X2 timber with two screws in the end can help turn a 'T' handle that's just out of reach
Or buy a suitable stop tap key from a plumbers' merchant.