I'm doing a major house renovation & have got to the stage where I need to move the incoming water pipe. I've exposed the pipe in question which is black plastic, I can't measure the diameter without cutting it open which I don't want to do until I can reconnect it all back up. I've measured the external circumference of it which is 68mm. I ***think*** that means I need to get 20mm pipe? Or would a house built in '79 ~ '80 era have an imperial plastic pipe?! Also, when they quote pipe sizes is that external diameter or internal? Thanks for any help.