You can add a UPIN in the basic details page. This then can populate the Unique Pupil Number field on the P&D assessment.
Add the field
The before you can see the UPIN on the P&D assessment, you'll need to add the field to the basic details page.
Open screen designer and add both the UpinTXT and UPINLBL fields.
Update the assessment
You can now update the assessment.
Open the P&D assessment and wait for the update message popup.
Click OK and the details automatically fill in.