I've read and used some valuable info. over the years on here so thanks for that. However I need some more direct info. if possible. I have an old but reliable Worcester/Bosch 2.4i combi boiler, but recently I've noticed a leak from the central heating flow isolation valve. I've tried Fernox leak sealer and a few others but alas no joy. So I imagine my next step is to replace the isolation valve. The question I am asking is do I need to drain down the entire system to replace, and would this be a special valve (as it is secured to the bulkhead plate with a nut and so may have a longer thread on one side) or would one from any plumbers merchant do?. Thanks in anticipation