Multisite subdomain install with domain mapping - SSL enable individual sites

Hi,

we have subdomain multisite install containing a few domain mapped sites. The servers have just been setup with a SAN multidomain cert. We'd like to enable and force SSL on one sub-site only - can you advise best method of achieving this? Would prefer a plugin solution for ease of management, but if this is not possible, we can go with .htaccess edits.

Many thanks,
Paul