Thanks for all the posts. I've done a fair bit of reading on the subject and its a bit of a mine field really, and as
@richardb70 points out quite correctly, there are issues with PCM and optical which I encountered. There is a lot of information on the subject and not all of it correct, or helpful either, but I did eventually get to have 5.1 through the optical with one of the more helpful solutions I found.
I've found his really is more of a Windows 10 issue than anything else. I understand this has been deliberately done for issues with copyright, licensing and compliance rather than a meaningless restriction on functionality. The receiver I got doesn't have HDMI which is unfortunate since just using an HDMI cable would have been an easy fix. I can't use the separate cables solution through audio jacks even though the receiver does have discrete channel inputs, because this messes with the setup for my DJ kit and Virtual DJ which I really took ages to refine, and have them how I like them. Not to mention its a messy solution which even the likes of Fred Flintstone would flinch at.
Having gone through all that trouble, I'm not actually sure it was entirely worth it. Films on my PC are certainly better, but I rarely watch films on it and when I do its more of a background thing. Music has no benefit, and even gaming improvement is... meh. I'll give it a bit longer and probably leave it in place now its there, but personally, I'm not initially blown away by the results.