Thank you guys!!!!
I am pretty amazed that 4 engineers may have missed this. Not only that but the pump that they fitted is now leaking. A bit of a disaster all round!!!
British Gas have now been four times to my house. I have a Potterton Puma 100 combi boiler. It was new in 2000.
On the display panel (within the boiler once the case has been removed) there is a red LED that is always on. Now my 'fault diagnosis' manual that came with the boiler says that...