Seeing as my original question (instant heater or electric shower http://forum.buildhub.org.uk/ipb/topic/1873-electric-shower-vs-instant-water-heater/) ended up being answered with 'neither- fit a UVC', I decided it would be better to start a new thread.     I know that these units are not a DIY install and that you need a plumber with a G3 certificate. They are also to be checked annually.   Just how much of the install needs to be done by a plumber? Need to know so tha