For reference, in case anyone else gets a similar issue:
Using the SES import facility to import a Keil MDK v5.25 Project.
After fixing a couple of other import errors, the link fails with:
L6031U: Could not open scatter description file $(RelLinkerScriptPath): No such file or directory
Full details on the Segger forum: https://forum.segger.com/index.php/Thread/5377-Keil-MDK-v5-25-Project-Import-Fails-RelLinkerScriptPath/