OK, got it now. Must have been a bit tired on Friday and in idiot mode!
FWIW I didn’t read the instructions carefully enough and was assuming that the data put on the query string by FSCF would automatically be readable by FSCF if I didn’t change the field names, but I can see now that for a number of reasons that’s a poor assumption.
You could make the instructions a little clearer on this (for other idiots), perhaps by underlining the words:
The redirect URL will likely have different names than the names this form uses, so this feature allows you to transform the names to match.
as you will certainly have to rename the fields, if only to prefix the form number that you are populating.
Thanks for the help.