From Muppet Wiki

Joan Sanderson (1912-1992) was a British character actress who played Dorcas in The Great Muppet Caper, opposite John Cleese as Neville.

Sanderson had previously appeared with Cleese in Fawlty Towers, guest starring as a selectively deaf guest, and appeared in Upstairs Downstairs, Rising Damp, and Wodehouse Playhouse.