Sue also pops up in a black and white Diane Rigg "Avengers" episode,with a black wig,as soon as she speaks you know its Sue!
I don't think it was a wig. She's naturally dark.
Sue had a bit of an Avengers history. She was Hannah Wild - Steed's Avenger girl - in the stage version from 1971. She beat Joanna Lumley to the role, who was devastasted. Joanna would get her chance a few years later of course.
Sue was also at the 1993 Avengers after launch party. See her here being greeted by Tara King herself (Linda Thorson) "Suuuueeee!"
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