Fixed it now - I had copied the proxy syntax from the nginx.conf.default example of proxying to apache, but it seems that the following lines are also needed in the conf for Nginx on port 80, otherwise the Nginx listening on the other port doesn't receive the domain being accessed:
[code]
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header Host $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
[/code]

Dave