Unfortunately, the new Photos application on MacOS does not support manipulating the location data when the GPS info is wrong or missing.
After a little recherche, I came up with Exiftool
Very nice, needs to be accessed via the terminal / command line. And wow, does it have a multitude of options! Took me a while to peruse the manual to come up with:
exiftool -n -GPSLongitude='7.85' /Users/bach/Desktop/IMG_6728.JPG
exiftool -n -GPSLatitude='48.0' /Users/bach/Desktop/IMG_6728.JPG
The "-n" argument means that the location is given as decimal fraction rather than minutes etc.
I hope this helps whoever wants to tackle similar problems.