OMG I've become a 'nimby' (not in my back yard) even if its not strictly 'mine' ..

There's some undeveloped land at the back of my parents house, which they've used on & off over the years as extra garden, so its been cleared (I know as I did most of it over the years), but they've never put a fence around it or such, now somebody has bought it (the next door neighbour) & intends to build himself a new house to retire in - trouble is it'd almost certainly be placed such that it overlooks my parents home (its on a sloping site)..

So I want to know how we establish boundaries (this land has never been accurately surveyed & the deeds are suitably vague on the subject) & 'anything else helpful' as really I haven't got a clue about the legality or otherwise of all of this, so somebody who has experience on the matter would be very handy indeed

Anyone ?