Contrary to the above answer, the title deeds will not tell you exactly where the boundary is. Yet another urban myth dispelled.
The title plan linked to the land title at the Land Registry only shows the general boundary.
The neighbour is wrong and the boundary is wherever it has always been, unless the two of you agree to a minor variation to the contrary. You have no obligation in regard to the hedge, nor to the fence unless their is a specific requirement for you to maintain that fence.