An Up Market Planter

This is indeed a Roman stone sarcophagus -  abandoned rather incongruously in front of commercial premises in Phoenix Way.

Unknown numbers of the dead lie somewhere beneath the earth in this part of Cirencester:

See:
Cirencester Excavations II: Romano British Cemeteries
Current Archaeology, 10th July 2013 [Discoveries on the Bridges Garage site in the Tetbury Road]