My /uploads folder is massive and full of sh!t that has accumulated since I first setup IPB WAY back in 2003. 

Can anyone give me any pointers to help me work out what is redundant that can be deleted?

My short answer is no.

What a nightmare that would be to go through the folder, and track down images that are no longer used. I know cuz I tried it! I was successful however once when for some reason with 3x version a default member avatar was created anew for each member who used it. So at one time I had 2 thousand avatars of the same image, that was an easy find and delete. But it spins my brain to think about figuring a way to filter and search for useless images and delete them.

EDIT: Please take my statement with a grain of salt, I realize how unproductive it was, I have limited skills in this area, my brain spun when you asked it thus my un-informed reply.