It's a high density plastic , usually solvent welded dates from the 70's copper shortage times . But I can't remember it's name - having used it just once back in the day . Durapipe ??? Google leads me to believe it's Durapipe abs.
Not to disagree with Nige who is an excellent mine of information but if it's the old imperial plastic pipe then it may be LDPE - Low Density Polyethylene - nasty stuff, as it it now tends to go brittle after the long time is been in and is a real nightmare to work with.
If it's 27mm OD then it may be the old 3/4" ID equivalent, 3/4" olives needed. Haven't seen original fittings for quite a while now. I'd honestly consider replacing it.
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