I want to join 15mm copper pipe to another pipe which is Imperial
polyethylene, Class C, low density to BS 1972/67, and installed around 1968.

This polyethylene pipe is of nominal size half inch, with an outside
diameter of 21.5mm, wall thickness a little under 3mm. It is black in
colour, and is to be used overground.

For the connector, will it be all right to use a metal (such as Kuterlite)
22mm to 15mm reducing fitting, with a three-quarter inch imperial copper
olive at the 22mm end going over the polyethylene pipe? I have a metal pipe
insert available that was left over from when the pipe was originally
installed in around 1968.