Website Insight would help with all of that Malcolm,
It does more than just find dead links, it lists all of the images it finds, all the pages it finds (if you do the full domain rather than just page by page of course), pretty sure it lists all files found etc that are part of the website. If you haven't tried it yet, take the trial for a spin and see what I mean. It will help you know what things are used and what are not by listing those it finds being linked to and used on the pages that are uploaded.
The other thing you can try, would be a time consuming thing of course, but would be very exact would be to use the HTML Editor and do a Find for a file, image, etc. and make sure it's looking in a folder not just one page. This would bring up any place it found that image,file,page referenced on any pages. You'd have to search them out one by one, but it would do it.
I'd take a look at Website Insight trial first and see if that will do what you need.