I have a push button cistern, fitted within the past 3 years, it began to run constantly into the pan. After trying a number of things, we think it is leaking water from the cistern into the pan because the large black rubber ring washer at the bottom is perished and not sealing. I can't seem to find a replacement washer - it is 80mm external diameter and 59mm internal diameter with a lip. I've tried taking the syphon to B&Q, Wickes, William Wilson and searching online, but I'm not sure exactly what it's called. There is no manufacturer's name on the cistern or syphon.