Our engineer was extremely polite and pleasant but unfortunately this call out will represent £90 wasted that we can ill afford for no tangible result. We did not think the fault had been isolated and we were correct - we called an alternative provider who found a further loop and valve tucked into the eaves of the bungalow, further round into the chalet roof than we have ever been and THAT was the problem. The engineer found it by very careful listening and tracking of an air lock. We now have heating and hot water again. We used SGS as they service the boilers at the school where I teach, but we will not be able to afford to use them again. Merry Christmas.