Thanks guys. So I've somehow managed to resolve the issue. First I tried pasting the front cover elements into a new document and exporting that single page. That too came out faded despite only having three or four elements on the page. I tried switching blend spaces as that thread suggested but that didn't work. In the end, I went back to my original file and removed any unused swatches. I noticed I was using an RGB white rather than 'paper' somewhere, which I assume came from a client vector I'd once pasted in, so removed and replaced that.
That seemed to solve the issue.