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