A few months ago my water started running hot and cold then after a few weeks my heating shut off I called out a plumber and he replaced the heat exchange this worked for about a month now it has started again and then the heating went I was advised to click the overheat button underneath which I did and the heating came on but water still hot then cold and if run for too long i.e. A BATH the heating shuts down..The plumber I used gone into liquidation..Can you give me any ideas what may be wrong so I don't get taken for a fool again??