After some playing about, i've settled on a redirect. If anyone wants to do something similar, create a new file in sites-available called subdomains, then do the usual sudo ln -s /etc/nginx/sites-available/subdomains /etc/nginx/sites-enabled
Inside the subdomains file, create a server block like thus:
If you want the subdomain to ignore the request parameter, then remove $request_uri? portion of the rewrite. This basically means that if someone typed mixes.35hz.co.uk/parameter it would ignore /parameter and still go to the relevant tag page without causing a 404. This may be more beneficial. So I'd recommend leaving that portion out. However this would be useful if you wanted a fast way to navigate to a user profile via something like user.35hz.co.uk/a_5mith. Because you could add the request_uri after /user in the rewrite.
Just add more server blocks as required for each rewrite. You can stack server_names so that multiple subdomains go to the same link, just add another server_name underneath the existing one.