You don't describe it or say where it is leaking from, but I will guess that it is a ball valve and has a screwdriver slot and needs a quarter-turn from on to off.
You can buy them for 90p if you don't mind replacing them when they leak, and repairing the water damage.
Or you can save up a few extra pounds and get a Pegler. A full-bore one will give better water flow.
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