Do you know if the restaurant has been given a long leasehold interest by the Landlord or whether it has a business occupational lease or none at all as it is the Landlord's business? This may be findoutable from a copy of the Landlord's title which can be downloaded from the Land Registry as there should be a schedule of leases that the Landlord's titl eis subject to...
http://www.landregisteronline.gov.uk/
If the restaurant has a long leasehold interest then it may be you could get a copy of their lease (costs about �20) to see what is in it and potentially what may be enforceable.
It may be that a formal lease had to be granted, especially if the landlord has a mortgage. If it is just a business lease it will not be registered though.
Hope that helps, let me know if you want to clarify anything x