Ark gives a good overview. But I have a few questions?
You said "partner." Are you two married? There is a reason I ask.
Second thing is this. If you are in NC, then you should be in Plan B. NC is during Plan B. Did you do a good Plan A? Did you give it time to work? If you didnt, then you need to Plan A him (read up on it here and in Harley's books).
If you did a good Plan A, then it is time to go to Plan B. That means write the letter (Plan B letter), send it...and then NO CONTACT!!! The letter will spell out why you are doing what you are doing. If he asks or tries to leave messages on why you are doing this, then just send him another copy of the PBL...and remain dark.
NC is for YOU!! Plan B is for YOU!! Plan A is for him. Once you are in Plan B, do not vascillate. I have seen too many people on here extend their pain because they give in. Do not do this.
So, my point is...make sure you have done a good Plan A for the period of time outlined by Dr. Harley. Then go to Plan B, also as he outlines. If you do, you will have good success.
One more note: Never, ever give him permission to do ANYTHING with the OW!!! he is a married man and does not have permission to do anything like that with anyone else but you. Never condone immoral and in some places, illegal behavior. If you do, if you tell him that he can go find out what is what with the OW, then he will!! If he was a drug addict, would you tell him "Honey, if you need to, you can continue to use drugs and find out if you really want to continue with them?" Then why would you give him permission (which is what you did) for him to continue his addiction to the OW?
Time to get with the gameplan here. Dont try to make this up yourself. these principles are tried and true. They are your best hope. You do it yourself, and you most likely will mess everything up.
In His arms.