As part of an ongoing bathroom renovation I am using a piece of 15mm chrome pipe as the cistern inlet feed straight down through the floor so it will be less conspicuous compared to the way it was before . The 4" waste pipe is just below this almost up against a joist so there isn't much room below the boards at that point either behind or above the waste pipe. I was thinking of using a short section of 8mm copper at the bottom end of the
15mm to get past the waste pipe then connect it to the mains which is on the other side of the waste . As a temporary measure until I finish boarding the wall behind the cistern I have done just that and so far it seems to work fine ....I replaced the inlet valve as well so I cannot compare how it filled before with now but it seems to fill up pretty quickly . Any reasons you can think of why I shouldn't do this .??Stuart