Complex sentence: When Hammer Film Productions came to make a film version of "Quatermass and the Pit", they didn't feel that she was quite the actress that they wanted and cast actress Barbara Shelley, who was taller and more fitting in with the Hammer image.
Split into simpler sentences: When Hammer Film Productions came to make a film version of "Quatermass and the Pit", they didn't feel that she was quite the actress that they wanted. Hammer chose actress Barbara Shelley, who was taller and more fitting in with the Hammer image.