I came home last night to find a flood in one of the downstairs rooms
and have spent the first half of today opening-up the floor above and
fixing the problem.

When I re-plumbed the house a couple of years ago I used plastic pipe
and Tectite fittings everywhere accessible; but where I couldn't avoid
having joints under the floor I used compression fittings because I
didn't totally trust push-fit. The joint that failed was a compression
elbow on the end of a long straight DHW run - the pipe had completely
pulled-out of the elbow. I presume it was thermal expansion of the long
straight run and an inability of the other "limb" to bend sufficiently.
I've replaced the compression elbow with a tectite elbow and arranged a
longer "limb" on the other side so hopefully there's more flex.

Moral of the story: take expansion seriously and trust push-fit.

Dave