There is a hedge and ditch presumption and that is that the boundary is on the side of the ditch away from the hedge.
This is because the landowner digs the ditch on his own land and puts the dirt from the ditch along the edge and the hedges planted in the spoil from the ditch.
That is exactly what you are referring to.
If you agree what you are doing between you, then to avoid arguments in the future, you could simply get a deed drafted with the plan which shows that the ditch is yours along with the hedge although to be frank, I probably wouldn’t bother and if there is an argument in the future I would just refer to this correspondence and common law presumption.
A simple letter between the two of you would probably suffice.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have