Thanks for your replies. I discovered the fault in the end. It turned out that two things were happening at the same time as the RCD was tripping ie the gas valve was opening and the ignition spark was sparking. Having fully checked the spark lead was not visually tracking I looked at the gas valve which is situated at the top of the boiler just below the cold air intake. During this cold period, water was condensing in the short plastic air intake pipe and dripping onto a plastic cover over the electrically operated gas valve. This cover is designed to keep the connections dry but SOMEONE had put the cover back around the wrong way leaving a screw hole in the cover directly over the terminal which was now wet and shorting to earth. I dried the terminal and replaced the cover the right way around and all is well.
I went abroad in early December to find that the RCD had tripped whilst away melting my freezer- we had a very cold spell then too so that explains why.
Cold damp outside air condenses slightly in the air intake pipe causing the problem on both occassions.
Thanks for your replies guys. I hope that this information will help someone else later.
Neville Hobbs
January 2009
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