Grandad666 chances are you have a blockage at the tee where central heating water enters the cylinder and also connects to the expansion cistern in the loft. I will shortly be off with my good lady to stand by the sidelines while she shops for things I have little interest in. So, if you are keen to get to bottom of your malfunctioning hot water system, do write back and I will endeavour to assist you without 'boilerman' attitude.