Unable to contact landlord

[Guest]

We're buying a lease on a property, but neither us or our solicitors can find the building freehold landlord to establish some facts. It is the ground floor of a building and there is a tenant upstairs who seems to know nothing. The owners of the lease we are buying have overlooked all sorts of issues and we cannot find out whether there is buildings insurance, who to contact about alterations to the property and so on. Mail to the freehold landlord has been returned 'not known at this address'. How do we proceed? How can we find the landlord?