The starting point, and really the only point, is what does your title plan and register say?
The title plan is, counter intuitively, not determinative of the exact position of the boundaries under the ‘general boundaries’ rule; essentially, unless the boundaries are specifically fixed by agreement, the plans only show the general area in which the boundaries are located:
https://www.gov.uk/government/publications/land-registry-plans-boundaries/land-registry-plans-boundaries-practice-guide-40-supplement-3#general-boundaries
However, you can generally reach a reasonable conclusion with reference to the registered title plan. I’d also take a look at the sale contract and the transfer which may include provisions for the developer to alter the boundaries within a specified time.
If they have sold you the land, though, they can’t just grab it back because they’ve cocked up.
This, absent a saving provision in the contract, and even that is potentially an unfair contract term if it allows the developer to deliver something substantially different than that originally contracted for.