The land registry digitised its maps some years ago, and now as properties are being sold this issue can arise. The new land registry map is legally effective, even if it goes against common sense, or long established borders.
To have it rectified you will need the agreement of the management company. I doubt you will be able to draw down the mortgage until this is done.
It is of course the sellers responsibility, but if you want to buy then maybe it falls back on you.