It's the hot water diaphragm thats torn. Unfortunately on some of the cdi range access to the diaphragm is a bit of a nightmare so unless you are a plumber with knowledge of boilers or a gas engineer I wouldn't attempt it. I've done this job a long time and replacing these is not an easy job even for me sometimes. you have to remove a few other components to get the water section out to replace the diaphragm and you run the risk of causing a water leak on these other components when replacing it all. A shame really because when you get the water section out replacing the diaphragm is quite easy. Worcester rectified this with later models whereby the diaphragm is dead easy to get at and even later models don't have the rubber diaphragms. In fact very few manufacturers use rubber diaphragms nowadays thankfully. Hope this helps.
