Hi Nic,
We turn the burner that the tray is directly completely off once its hot otherwise it'd burn the bottom out of the meat & we certainly wouldnt want that!! Ours is a 6 burner BBQ so we dont have all the burners on to get it to 180 degrees.
After reading the other posts I'm keen to try the rotisserie idea too... Must hunt one down for our BBQ.

Talk about mouth watering!! All this talk about roasts has made me starving... I'm off to get something to eat