Sounds like there was s mistake made when the plot was originally sold from the developer to the first purchaser.
Often more senior lawyers will do the site acquisition and deal with all the documents such as s.38 agreements for road adoption which are usually backed by a bond.
More info on them here...
http://www.manualforstreets.org.uk/uploads/cha pter%2012.pdf
See if you can get a copy of the s.38 agreement, if there is one, from the local authority planning department. There should also be plans in it.
There may also be provisions in the original transfer or lease to the first purchaser relating to the proposals for road adoption.
More junior people then usually do the sale off of the plots.
Plans will have been drawn up by architects to the required scales for Land Registration purposes for the separate transfers or leases of the plots.
It's likely the plans were drawn up without proper reference to the proposed adoption of the highway in error or not properly marked up when being reproduced for all the leases.
It would be interesting to see which was completed first, the transfer or lease (date it was completed rather than a term commencement date) to you or the adoption agreement.
Often a new development will have private roads with plans for adoption at a later stage.
Obviously your property has been valued with the additional land and, I would imagine, yoru mortgage valuation done on it (whether there is any effect depends on the amount of mortgage you took out against the property).
What would be gained by having a footpath made up? Would it serve any purpose to anyone?