Black plastic (polyethylene) is from the 80's, so the first replacement for the old lead pipe, but ours was 25mm 'ish, so the old 1". I suggest a trip to a good plumbing supply company who may have a solution and they should be able to sort you out, one way or another. The norm now is a thinner wall with an insert to support the pipe when a compression fitting is used. 20mm in the UK is a new one on me as the normal size would be 22mm.
It's 20mm here in France, but the inner diameter is 16mm and an insert is sometimes built into the fittings, because the pressure is a whopping 10bar. So you could get a 20mm connector, without inserts, from France if necessary?