Who played Mercury on the series As time goes by?

Answer 1

There was actually no actress that played the character "Mercury." As we find out by allusion, she was just a fictional character made up by Alistair to cover the pain of his rejection by Judy. The tall blonde woman that passes by the table and that the group at first presumes to be Alistair's "Mercury" is jokingly referred to as "the plain girl" and is probably who the original poster of this question is asking about. I too would like to know who she was/is.