Question Author
Found a local plumber on mybuilder.com website. Plumber came and replaced the ball valve but the problem remains exactly the same. Not pleased. Got charged £60 for the job, including parts. The plumber now says that he has to purchase a new 'syphony' thing as the seal at the bottom has bubbled and is not sealing properly. (Have a top flush dual chamber). I am told that it costs about £20 to £25, plus labour.