Thanks. If I am not mistaken, the ATR72 does GCI-SOU, so the four ATR42s share JER-GCI/LCY/BRS between them.
One does five GCI rotations, the second BRS plus the first/last GCI/JER of the day (overnighting in GCI) and the third does LCY. So indeed there is one spare - plus two sitting idle on the ground at JER for a number of hours each day.
Good that SI operates some written down early ATR42-320 that allow for such a lenghty parking exercise.
One has to wonder if they are looking for a noon-time flight to somewhere in the south of England to keep the BRS aircraft a bit busier.