There are several possibilities:
1) The land is owned by another landowner
2) The land is a public right of way, but not the public carriageway - it is instead a byway, restricted byway, bridleway (or even a footpath). In which case you probably have long usage rights to take a vehicle up there. See if it is the register of rights of way (County or Unitary Councils typically hold these).
3) No one owns it - it has just become lost in a time warp.
Does your land registry title make any comment about what right you have to pass / repass up there to access your land? - this may give you a clue.