If you look up error code 102 in FileMaker help, it means "field is missing"
Script steps that start with the word "insert" do not work unless the specified field is present on the current layout.
You are correct, this is the puzzling thing. I should have said that the field does exist.
But does it exist on the current layout?
It does indeed, it exists on the Student final PDF storage layout.
Enter layout mode and click to select that field
Check to see if Browse mode access is permitted.
Check the text found in "Display data from" to make sure that it is exactly:
Students Details::Student Report 1415 Autumn
You were right, I had made a schoolboy error and selected the wrong database field.
I must have checked that 4 times!
Thanks for your help.