I think you'll find that this was I./SKG10... according to the excellent "Luftwaffe Hit & Run Raiders" by Chris Goss [Classic, 2009], both Portsmouth & Plymouth were targeted between April & May 1944, with mention made of a unit loss during a raid on Portsmouth on the 22nd May... his is an active member here so I imagine he'll be able to elaborate further...
Cheers
Peter D Evans
LEMB Administrator