I called in HomeCare who sent a plumber around. The plumber in turn told me that the the leak is coming from the open vent on the heating, so I needed a British Gas engineer. I was promised he'd attend between 3 and 6 and that I would get a call when he was on the way as I only work five minutes away. When I got in there was a card through the door saying that the engineer had called at 5.03 when I wasn't home.
I called back and am due to get another visit between twelve and six today. It is essential it gets done as I've had to cancel the electrician for today and re-book him for tomorrow. I'm about to call and stress this to them which explains the collection of thoughts on these spindrift pages.
This sorry tale also finally explains why the bathroom roof is waterlogged. The vent should be empty. If it is leaking in the loft this explains why the overflow tank would have been continuously streaming water onto the flat roof, via an amazingly stupidly sited pipe.
Update: Today's job number is 138 745 2774