What have you done to establish who owns the land?
You have examined all of the usual avenues of enquiry. It is therefore unregistered land (not showing on the land registry's registers) and therefore it will be difficult to establish who the owner is.
Will the land in question become 'highway' once/if developed?
I have looked into this further. Highways will not appropriate the land and will not consider any application where ownership is uncertain.
Unless you can establish who the owner is then it will be an issue and difficult to proceed.
An option is to fence it and try to claim possessory title but that is a 10 year wait. Although the owner may surface during this period but the land will become a ransom strip and the paper owner can effectively hold you to ransom.
Without knowing who owns the land you are stuck.