I managed to get the SSL working using the current SSL key/crt files.
As I had no option to re-create a new CSR/CRT, as this would have cost.
I put the current *.crt and *.key files into the sites /ssl/ directory on the ISPconfig server.
And enabled 'ssl' in the site options.
This worked, and ispconfig created the following entries in the apache2 site file: