Land Direct advise you don't rely on their maps to establish a true boundary, so just be cautious there but in most modern estates, it's usually easier to establish.
Before you go down the legal route, figure out what you want to achieve. A solicitor's letter may well be enough to prevent them going any further, but if they remove all the trees, you may be able to force them to reinstate a boundary in the current location, but they will likely have a say on the form that boundary takes.
If you do go legal and want more than a letter to act as a warning shot, they'll advise engaging an expert to establish the true boundary, from there you're talking a civil action to seek reinstatement of the boundary potentially also with compensation for loss of the trees, though consider what fencing off that area might look like to a judge.
Looking at the pictures, it does look like the boundaries should indeed follow the well established hedge line.