This is where a standard garden hose, clips onto the pressure - like a
normal male Hoselock type garden hose quick connector.

It looks identical to the male connector adaptor, used on a threaded
garden tap, except is has a slightly larger internal thread on the
plastic adaptor. It seems to be around 1mm larger internal diameter,
than the standard garden tap thread. Anyone know what I need to be
looking for please?

For now, I am trying sleeving the inside of the plastic adaptor, with a
used bullet case, glued with Araldite. The bullet case has a taper,
which almost matches the bore of the adaptor.