Another thought: Tap connectors usually have a shoulder or spigot on which
the washer sits, and which is intended to go inside the tap tail. I have
somethines found with plastic fittings - particularly on toilet cisterns -
that the spigot *won't* go into the fitting, and needs to be filed back to
less than the thickness of the washer in order to get a seal.

Yes. Presumably the plastic pipe is too thick. Shall I just go for a 3/4"
iron and seal it? One of the main disadvantages of the tap connector is the
much reduced bore through the middle, so a standard iron fitting should
allow more flow.

Obviously with non-tapered threads, you just need to know when to stop, but
it should seal fine against plastic with PTFE shouldn't it?

Christian.