Applying for adverse possession on disclaimed Bona Vacantia land

Hello

We would like to apply for adverse possession over a strip of land on the side of our property that for the last 20 years we have used and believed was our own. It encloses a gate, our side door and entrance to our back garden. It is not a right of way, there seems to have been a mistake when plans were submitted to the land registry in 1999 about where the boundary fence was erected by the freehold company who bought the surrounding land.
They erected the fence in one place (enclosing our side path) however the plans submitted show that not to be the case.

We did apply to purchase this land under from the bona vacantia department but unfortunately the strip of land forms part of a much larger plot and we have been told that the Crown cannot split the land to sell a small part.
As part of that process there has been a disclaimer placed by the crown. Does this now affect our ability to claim adverse posession of this land?

We bought the house in 2003 and can prove the boundary line was in place when we bought it and has not been moved by us.

Thank you.