I'm looking for a Harry Potter fanfic that I last read probably five years ago.
Severus and Harry have an intimate relationship, but I'm not sure if the relationship starts as friends and builds throughout the story, or if they start as lovers. I do believe they either have a cover as married, or actually are married for a good portion of the story.
They are called before the Queen of England to be rewarded for killing Voldemort. Neither one want to go as they are both very private and just want to be left alone. I'm not sure if she knights them or if it's discussed, but I think something along those lines happens when they meet. They meet her in a court room.
MI5? (possibly a wizard division instead) has great respect for them (especially Severus) as spies and asks them to help with an investigation. The investigation has to do with potions being made under the guise of Muggle medicine.
Severus' cover is as some sort of Muggle research doctor. There is a scene where he is touring the labs the potion is being produced in. I'm not sure what Harry's cover is, possibly as Severus' assistant.
At least one of them ends up tied to a chair near the end of the investigation.