I've bought an instant type water heater (like an electric shower unit) from Screwfix. The inlet and outlet are plastic and threaded for a normal 15mm compression fitting (one nut and olive supplied) except the copper pipe can't be inserted into the fitting which means the olives have to be at the very end of the pipe. This doesn't seem to work - very difficult to make water tight and both pop off when another tap on the line is turned on/off. Is there some sort of special fitting like a tap fitting, which will do the job? A tap fitting won't do it because it has a short spigot, which I suppose I could file off. Or should an olive at the tip of the pipe work OK and the problem is my plumbing technique? Or should I attempt to drill out a bit of the plastic so that the pipe will go in? Or should I send it back to Screwyou? But I've already put in the cable, pipe, RCD etc.
cheers
Jacob