To sum up, the roof is different (exhaust vents and walkways) and the ends are different (marker lights) on D833-865.
D803-12 differ from the rest of the class in having different side grilles (two-piece not three) and covered hatches below the solebar. These details didn't bother Bachmann. D866-870 lack the grilles above the nameplates which the others have on one side. D870 also had horns mounted above the cab. D818 had a different train heating boiler (different roof vent) and D830 had a different engine. D832, as mentioned before, had Class 43 style roof walkways.
Geoff Endacott