On Mon, 28 Nov 2016 01:26:40 -0800 (PST), David wrote:

I agree entirely. From the sound of it, though, the OP (and
British Gas) are well on the way to eliminating the likely
scenarios, so I thought I'd chip in with something more unlikely


That's exactly right. BG are not idiots, they know the likely
causes, and indeed we have been working to eliminate them one by
one. The trouble is we are approaching the point that serious damage
will have to be done to the flooring to pin point the leak and
repair it. There is some insurance coverage for tracing and repair
but there will be a lot of disruption to carry out the work.