This is one of the early series, when they seemed to put more effort into getting things right and being very "John le Carré", convoluted, wheels in wheels, everything is ambiguous, etc. So I would be surprised if it were a simple mistake.
As I hear it, Harry introduces Tessa as "Miss Tessa Phillips" and the minister says "How do you do, Ms. Phillips?" (I think). That would figure: as a minister, he is being politically correct and not recognizing her marital status, while Harry is being old-fashioned (and letting the minister and the audience know that she is unmarried). Letting the minister know that she is unmarried might have something to do with Harry saying to Tessa earlier "Stroke him. See if he meeows" (more ambiguity).
Tessa mumbles "Very well, thank you, minister", which is, indeed, odd.
Of course, she is - unbeknownst to us at the time - actually his mistress and he is only pretending that they are strangers, so it could be that she avoids the standard reply "How do you do".
- This could be to indicate that she is flustered (not very likely).
- Or it could be (I've forgotten the technical word for this) a clue as to what is to come that the careful viewer may notice (and think "Why did she answer like that?", "Have they already met?" etc.).
- Or he is pretending that they have never met but, as an experienced spy, she does not answer in a similar, undeniably deceptive fashion (because MI5 may already know or later find out about the affair).