I have a leak under my kitchen sink. I need to replace the stopcock but have been unable to turn off the external water supply as the outside water connection that switches the water off is a circular piece of plastic of about 3 inches in diameter with six little prongs coming out of it. This item is too tight to turn by hand and we are therefore unable to switch the water off and replace the stopcock. We have bought a new stopcock. Can you advise me how I can get this fixed or where I can get a tool to switch the water off.