lmao, a HM Land Registry lawyer got back to me about the proposed alteration to our land title, where they described the alteration in terms of coloured regions on a map but sent me a black and white copy, and their tone is very much "oh for feck's sake, what eejit did that?"
anyway it turns out that, as it stands, we currently own our house and the land under half the damn street from the looks of it. the builder royally messed up the paperwork, and HM LR clearly wasn't paying attention.