Thanks for your reply Mark. I have not been able to find separate httpdocs and httpsdocs directories on the host, so I don't think they are split.
Just a quick update.
nuan88, you could be right re. the hoster (but I hope not as that is going to be a pain).
This morning I did the following:
- Created a test subdomain (different server and hosting - just to be safe).
- Added a new SSL certificate.
- Installed a fresh MODX installation (latest version at the time of writing was 2.6.4-pl)
- Used @nuan88's .htaccess file
- Set server_protocol to https
- Enabled friendly_urls
- Created a couple of dummy templates and pages to check linking works etc
And it all worked!
Now for the hard part of tracking down what the differences are. It looks like I am going to have to (re)install MODX on top of an existing certificate.