Thanks for your replies.
My main reason for wanting to clean it up is that is confusing to read and decide which CSS to use and some of my old CSS is suspiciously puzzling!.
I like DrDocs approach. This is the kind of thinking I will use on future sites. Regrettably I did not have his knowledge or foresight when I started using CSS!
I am tidying up my site as I want to make it responsive. This will be easier and less bug prone if I know what CSS is actually being used.
The background situation:
10,000+ page site built over 10 years. Desired visitors visit multiple pages. I'm getting rid of all inline styling as that'll make it easier to change to a responsive design
Google Developer tools checks which CSS is used on a page, but doesn't seem to do it for a set of pages so is not really a help. [In case anyone's reading this and wonders how to do this, in Chrome hit ctrl-shift-I to open developer tools. Select Audits, tick Web Page Performance then run the audit to get a report including unused CSS].
I'm surprised there isn't a tool that will automate this tedious chore :-)