Speedfit do a single piece plastic reducing fitting for that:
http://www.bes.co.uk/products/154.asp code 10513, just over a third of
the way down the page.
My own preference is to use brass compression fittings for mixed
copper/plastic joints and a good plumbers' merchant will have 22/15mm
reducing fittings for less than plastic:
http://www.bes.co.uk/products/141.asp code 6714, about a quarter of the
way down. Don't forget to use a stiffener insert for the plastic pipe.
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