Chances are you have a high pressure valve & possibly need to change to a low pressure version. 1st you need to find out if the WC is off a tank in the loft (Low pressure) or mains the stop cock. Turn off your main stop cock & flush the wc a few times. If the wc is still filling you wc is off the water tank in the loft & is low pressure. You will now need to turn off any stop cocks from the main tank & make sure the wc is not filling. (Also have a look & see if there is a local isolating valve for the wc. If so! Just use this) Now you can need to either buy a new cistern ball valve which is about £10/15 or just the little cone valve that fits inside 50P. most new valves will come with a low or high pressure insert. That is either red or blue. But before you change anything, strip down the ball valve & make sure its not got muck or scale inside blocking the flow.