Personally(coming from someone who has had a lot of interviews)
I would not send a thank you letter. This could be deemed as kissing a**e KWIM?
I would say wait intill the Fri for a response and then if you hear nothing, call them on monday.
That way you are showing how intrested you really were, but not in a pushy way
and if the vacancy has been filled by someone else, they may put you on file for future jobs.
Good luck. I really hope you get the call saying YES!
Sarah