Warmplan xx LPG to Natural gas?

I have a brand new in the box Robinson Willey Warmplan xx auto heater, it was originaly going to be used with LPG. However due to a change of plan I would now like to run it on natural gas from the mains. I see in the instructions on page 2 of the installation and servicing booklet that the injector for natural gas is part number N9 and the injector for LPG is LP5. Is this a simple matter of buying a N9 injector and having it fitted ? or is it much more complicated. Many thanks in anticipation, Vincent.