The biggest thing, I've just found actually, is that almost all my lights were spot lights and I had them all acting as Mental Ray 'area lights'. New territory to me so not fully understanding the difference, but, unchecking the area light option in the mental ray tab for all the spot lights has dramatically cut render times.

Hope your hiatus has been good.

(p.s for the GI version of the scene Ive cut about a quarter of the 'non-direct' lights out.)