Hello Ashleigh!
I wonder how things have gone with your trench? We have a no dig garden built according to recommendations and the couch grass is proving very exhausting. We have decided to do as you did and then at the end of the season we will add new cardboard and compost and cover with black sheeting for 28 months. It feels imperative that we stop the couch grass entering from outside so I hope you have good news!!!! Thanks