Recently we had a water leak from an old and inaccessible compression
fitting underneath the bath. As a quick fix I cut-out the section of
pipe and fitted a 22mm push-fit stop-end until I could replace the pipe.
Today I was starting to make-up the new pipe and fittings when I noticed
that there were a few minor drips beneath the stop-end and that a
yorkshire coupling slid over the pipe end (having removed the stop-end)
was rather loose. Then light dawned - it was 3/4 imperial pipe! I was
very lucky that the stop-end stayed put under mains pressure. Now to get
an imperial-metric coupler.
Lesson learned: measure the O/D of old pipework and don't assume that
it's metric - if it's about 20.5mm it's 3/4 imperial.