Do not worry in terms of time limit you have 12 years to do something about it.
How do you know it was more than their "fair share" ?
You appear to be making the assumption that the centre of the hedge is the boundary which is not necessarily so.
I would get hold of a set of Office Copy Entries of the title to your property and that of your neighbour. These can be obtained from the Land Registry and any local conveyancing solicitor should be able to get them at a reasonable price , they are £6.00 each for the title document and map.
With each set is a scale map of the property from which the dimensions and boundaries can be ascertained.
From there , if he is encroaching, you advise him he is trespassing and he moves it or you sue and he gets to pay your legal costs when you succeed.
You are welcome