How to clarify property boundaries

haz

Imagine a four-sided property (house, garden, driveway) in the midst of a line of identical properties. The four-sided properties are at a slight angle to the main road, i.e. not rectangular with right-angled corners. At some point in time the owner builds a garage at right-angles to the frontage and subsequently sells this rectangular property, leaving two triangles at either side in the original property - but the sale deeds are in precise foot-and-inch measurements, as are the original property boundaries. Then the owner sells "the property" (Number X, on Y Road). I assume that the new owner has purchased the entirety of the original plot, less that which has been sold, including these inconvenient leftover triangles.

Now the new owner suffers the potential encroachment upon the entire remaining garden where the neighbours either side might imagine the edges of the (newer) garage define the property boundary, and cut (say) two feet off each side of the property - perhaps by parking a vehicle in line with this wall, then adding some garden pots, etc.

How does the new owner a) establish the precise ownership of "the property" and b) convey this to the neighbours with least cost and dispute? Imagine the land registry officer had helpfully drawn around the property in sale in red ink at the time of purchase, ignoring these two triangles.

I assume that returning to the conveyancing solicitor would be a good first step, and that the ownership of the two triangles would not be in serious dispute if dealt with promptly.