In the absence of any replies, I decided to purchase an additional ip for my server and move the domain to this new dedicated ip.
All very straightforward to do using Plesk panel.
The only glitch was that the new ip wasn't listening on port 443, so I was getting "connection refused"...
After some reading on the above subject, I'm still left a little confused and would like some advice.
Plesk Panel 11.5.30, on Centos 5.10.
I have 2 'domains' running on the same ip; sub.mysite.com and mysite.com.
I need to add SSL to sub.mysite.com.
From the documentation...