What a faff.

Following yesterdays deluge from a burst pipe I investigated further. But not easily. The up pipe, a 90 degree turn and a straight bit (basically the right half) had all been encased in a box fitted to the garage. The straight bit protruded through a hole in the box. Then they fitted the left hand bend, a down pipe and finally the tap. So impossible to extract without demolishing the box or the pipe work. No easy task, but eventually the pipe work was removed. So I was going to just remove the old joint and fit a new compression joint. Then something in the back of my mind (I think a comment from years ago from blipper Porridgewog) that a non return valve had to be involved. So shopping tomorrow to get the bits and assemble and fit. A new box needs to be built,  although I have toyed with encasing it in a length of guttering filled with squirty foam to insulate it. 

Copper pipe and fittings £20
Tap with non return valve £12
Visit to Screwfix to buy tools that I can't find in the garage although I know I have them. £30

Plumber call out to do the job properly and repair my damage £200

Job's a good 'un.