Hi Abitwobbly, I've been a lurker for a while, but felt that I couldn't not reply to your post. I will say, I have no experience with narcissists, but will comment from the getting over an affair POV.
About 3 years ago, I found out that my BF was having an affair with someone that I knew quite well. They'd been seeing each other for about 6 months - although I'm sure it was longer than that in reality. One night, out of the blue, she sent me a text to tell me what had been going on. I spoke to her, got some answers from her, thanked her (!) and have never had anything to do with her since (we had a few mutual friends, I cut contact with them as well).
I went straight to him, he admitted the affair and some of the details. He didn't know that she was going to tell me, so was caught on the hop so to speak. I actually went a bit crazy for a couple of days - refused to leave his house, switched between sleeping and drinking and shouting at him. Over those couple of days I got the answers that I needed - he volunteered plenty of information that I didn't already know. He also spoke to her on loud speaker to tell her it was over, and he was with me - whether I stayed with him or not. Without doubt, it was the worst few days of my life.
I eventually left his house, with the full intention of never seeing him again. I cut total contact for about 2 months, until I received a letter from him, basically explaining his state of mind at the time, his reasons (such as they were), what he'd learned and saying that he loved me and missed me and how sorry he was, but fully understood if I didn't respond. (I know - sounds like something from a bad chick-lit book!!).
I told myself that I didn't still love him, so it would be fine to contact him - just to see... rollseyes at self It took months, but we got back together and we moved in together about 6 months ago.
It's been a very very long road to trust, including (i'm sure i'll forget something):
Having full access to phone, email (incl work when I wanted), facebook. At the start, he had a phone that put messages into 'threads' and you couldn't delete individual messages (only the whole thing), so I could check if anything had been deleted easily.
Having full access to bank and cc statements - at one point I made him account for the cash he took out as well.
He introduced me to his family, more friends (of COURSE he was having an affair 3 years ago when I hadn't met his family!!) and work collegues (new job in meantime).
If I was having a wobble - usually if he was out without me, he sent me pictures or where/who he was with, or got his friends/family to text me to confirm.
Of course these things were done obsessively at the start, and less and less as time goes on. I've had crazy moments, it was two steps forward, one step back for a long time. But through all this, it's been his actions and attitude that have kept me here, he's never suggested that anything was my fault, he accepts total responsibility for his actions and my crazyness! I still open his bank statements sometimes, and occasionally check his phone (usually if i've had too much wine!). Do I trust him? Not fully, no. But I believe that I will never fully trust anyone ever again - I'll always have my eyes open.
Hope my ramblings help in some way, it CAN be done, it's bloody hard work but it CAN happen.