I suspect that as she didn't have to speak it wasn't worth having her on location, it's probably one of the crew.
Is she wearing a hat? It's difficult to tell in the picture.It's hard to say but I'd say with 60% certainty that other her, the hair does look different but the face shape looks right.
It doesn't look like Mrs Partridge to me. He also had a thing for Miss Moody. Maybe this was another woman he had on the side.