Raymond you are a legend! I have changed the field type to text in the database table mdl_certificate and it works!
With regard to the teacher printing, I had added the teacher to the roles in certificate before logging the problem. I have messed around with it some more and still no joy, but I can use the custom field instead, I have just entered the teacher name there.
Custom certificate (plugin)
Custom Text Field in Certificate GUI not Displaying
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