Any hints on how to close a very staff mains water tap?

Sorry, I meant a stiff water tap

Hi Janice,
You posted in the wrong area so I moved your post. What tap is giving you a problem? Be specific! What type? Where located? Brand name? How old? Is this a handle that moves up/down and around or a knob the turns. Please don't make me have to guess. Regards, Tom

Thanks Tom. It the tap connecting my home to the mains water supply. We've just bought the house - about 30 years old - so I gues it dates to the 80s. It has a big round plastic knob that should turn around. The name is "CONVAT WATER PATENT". Many thanks in advance, Janice

I need you be more specific about the location. Is this the house shut off located where the water service enters the house or is it on the main in the meter box?
I'm going to assume that it's the house shut off and that you simply have a stop that's frozen open. Did it look like this,(except for the handle) on a pipe leading into the house?(see image) If so loosen the top nut a bit and put a pair of pliers on the handle and begain to put pressure clockwise. When the handle begains to give a little you have broke it free. Snug the nut back up and you're OK. Good luck, tom