Seems I`m in error.
Looks like I took the Mention of R1 + R2 and assumed a ring wheras it was a radial. Silly me.
However, ring or radial, you must completely disconnect to test properly. However if you want to keep the cpc in the earth bar and temporary park the L conductor there too then, so long as there are no para earths skewing the readings, then fine. I think easier to have them all disconnected for these tests though and just link them with chock blocks or wagos etc to to the test measurements