I decided in the end to get a Ferolli engineer out to it. He used his sniffer device and found it was leaking gas from around the pressure test point on the pipe from the gas valve upto the burner. He tapped it gently and the test point fell off !. Looking at the two pieces, it appears the brazing only attached to the copper pipe at two points about 2mm square top and bottom, which left a 5mm dia hole in the side of a 15mm copper pipe fed at full pressure with gas. I'm amazed it hadn't caused an explosion........ the pipe cost £5 and labour £75, do you think I should pursue a claim for a manufacturing defect with it being 6yrs old now ?. I just keep thinking about the possibilties had it fallen off at say 6am as the heating starts up on the timer ............