If the Brit's are targetting Iraqi's firing at their own people with mortars, I'd be willing to bet that the artillary is being aimed by the Brit's own anti-mortar tracking radars. Usually, those radars are tracking inbound mortar shells, but it appears that there's no reason why they can locate the "start point" of mortar shells going sideways from the radar itself.
In a gun battle with artillary units, don't show up armed with a mortar.