Gaining Access

Access

Morning all,
Have any of you had any experience with a tenancy where you cant gain entry to a property? I have 3 terraces in a row and i have not been able to enter one of them to arrange gas & elec certs. Rent is still being paid on time. Looking through the windows (front and back) the house looks in great condition. People are definitely living there . I have been several times/knocked on the door etc and tried letters in the post (typed and hand written) to no joy...
I'm not sure what to do. Forced entry and changing the locks just so i have a set of keys is surely going to be a problem when the 'tenant' returns and then they cant get in etc etc?!

All advice will be greatly appreciated