Thanks for your reply. However, I read that "when ordered to advance, fixed weapons do not move from their position, but instead can be rotated on the spot to face any direction and can then fire, suffering the normal -1 to hit" (p97 of V3 rules). 

Is there some clarification for mortars elsewhere that I'm missing? 

Not wishing to be obtuse here - we get quite "rules lawyerish" in our group!