Brian Locking was in the Shadows for only 18 months. Fact.
John Rostill was in the Shadows for about 6 years. Subsequent bassists such as Alan Jones and Mark Griffiths have been hired hands working for the Shadows.
Many people felt that Rostill fitted rather well, including Brian Bennett, who should know. He has described Rostill as the Shadows' best bass player. He also contributed quite a bit of material.