Susan Twist first appeared as Mrs Merridew in The Wild Blue Yonder, then re-appeared as a background character in The Church on Ruby Road. In the script for TCoRR, recently released, the direction says the following:

"A WOMAN in the CROWD - a woman we’ve seen as Mrs Merridew in Special 2, a woman we’ll see a lot more of - YELLS"

In this instance the season is defined to end with the 8 episodes that will be in spring.