Hi. My gas boiler refused to start the other night. There has been an intermittent leak of water from it now for many years which was initially helped by plumber putting in some sort of sealant. Leak getting worse over last few weeks. Plumber due but on his last visit said boiler was getting old. Arent we all. Anyway in case he suggests a replacement what should i ask or expect ? I have heard that certain boilers can heat hot water instantly for showers, bath kitchen tap etc. What brand is best? Its a 2 bed apt. Any experiences welcome. I should have added the hot water cylinder is unvented.