I have always asked for and received the approval from the mortgage holder. Probably depends on the landlord and your rent though, eg. village house vs high end.
Technically you don't have priority if your landlord defaults on his payment and the bank repossesses the flat without the written agreement.
Most apartments have been owned through a company so the agent has always provided current land register and annual return for me and I have given an ID copy to the landlord. Address and phone of the landlord have always been on the contract.