Thanks for the input chaps. The scene is huge (approx 9k objects not including Vray proxies) I found a script which detaches faces based on material ID which has helped to be able to isolate the material ID's into objects. Now i know that each object only has 1 ID it should be simpler.
Anyone know of a script which will select all objects in a scene based on their multi sub object material ID? Selecting an object by material doesn't respect material ID's so you just get every object with that multi sub material applied to it. I would have thought its something people want to do often but having looked…