I forgot to mention when I replaced the parts mentioned it worked fine for 24 hours then the fuse blew so I went back out. I found that the air admittance valve on top of the boiler had failed causing water to get into the boiler. My guess is that as I'd drained the boiler to change the valve air got into the system, the air admitancea valve had dirt in it so when it opened to let the air out it did not seal correctly and caused the leek. It had not leeked before this as no air had been in the system for ages so it had remaoned closed