Greetings.
Many thanks to Joe for figuring this out.
The missing piece was to add the following to the ssl-global.conf file;
<VirtualHost *:443>
DocumentRoot "/srv/www/htdocs"
SSLEngine on
</VirtualHost>
This was added near the top of the file, just after the 'IFModule' and
'IFDefine' in the file.
Again, many thanks, Joe.
Regards,
Steve