Had to wait until today for the parts department to return from Christmas break - the accessory drive belt they gave me proved to be the wrong one. I suspect it is for a 2.7L TurboDiesel with no air conditioning. This assumption was backed up by the parts guy explaining that occasionally the ambulance vans may be ordered without AC, and it is added on later. Or it may be that he's wrong, and that the VIN and engine number do not indicate whether AC is present.
Either way, it was very obviously not going to fit, so into Ballarat. Made it as far as Clunes before realizing I'd neglected to grab the belt that was wrong, so I decided to grab both of them, which turned out to be a good idea.
Unable to locate the correct belt in their parts catalog, he resorted to taking them off the shelf and measuring them. Apparently the belts are the one part that Mercedes admit you probably needn't go to them to get it, as this was an off-brand one too.
Got home and got it installed quite quickly, and I'll keep the old belt for emergencies.