I think the fault is pointing towards the automatic changeover regulator I had fitted a few months ago. From the visual research I've done on these, mine seems to have been fitted upside down with it's vent exposed to the elements. As it is the clear screw on cap that covers the over pressure valve (pointing downwards in this case) has water in it. I can't help feeling that the thing is full of water and ice. Time to call the installer methinks.
David
February 2012