Thanks for getting back to me Craig.
Yeah, I found that info online, but I don't have access to the file system to edit or add code.
I imagine a lot of users on here are in the same boat.
I found a workaround however, with the help of our host. I'll share it here for users in my situation.
This workaround doesn't change the baked-in font, it gets rid of the phrase by using HTML.
In the 'Local customisation' box type no spaces, and save the language pack. You must have the permissions to update the language pack.
This inserts a blank where that string was.
I updated our template to include the same phrase in the font I wanted.
The template of the certificate is uploaded via Quick access menu>Plugins>Activity modules>Certificate>Upload>Borders section.
Thanks again - great community.
Jane