My main question is: does anyone know where I can get some kind of reducer 18mm-15mm. Or for that matter, anything that I can get from 18mm to any standard polumbing pipe.

Steel tube is also often referred to as poles, or pipes; which term is used usually refers to the application it is being used for. it sounds like old alkathene,will be class c or class d,these two types require if my memory is correct,different inserts.you should be able to convert to MDPE[blue pipe].If you can cut a section out,do this and then take it to your local Plumbers Merchants,hopefully theyll help get the adapters The steel pipe we supply is manufactured form BS1387 grade mild steel which is available in a red oxide finish (often used for gas and oil installations) and galvanised which is ideal for compressed air installations and external applications.