Grey waste drain tap broken problem

nedsram

Our motorhome must have reached that age where things start to wear out. Earlier ths year the handle on the clean water drain tap broke off. No problem there - simply screw out the tap and fit a new one. However today I could't open the grey water tap. Exerting what I hoped would be sufficient force to do so caused the fitting to snap off where the tap screws into the drain pipe fitting, leaving the male thread stuck in there.

I have obtained a replacement tap fitting, but need to get the broken off male thread out of the female fitting before I can fit it. Can anybody suggest a simple way of removing the broken in thread without damaging the fitting on the pipe leading to the waste tank please? Many thanks.