I was about to rent a flat that was advertised as having an EPC rating of C, but I checked the gov.uk website and found it was actually an F, therefore illegal to rent out. I queried it with the landlord. He said that was an old EPC done prior to work carried out, so he got someone round the next day to do a new EPC, and now it's rated D. However not much has changed - the proportion of energy efficient lightbulbs has increased from 75% to 100%, and TRVs have been added to the radiators, nothing more. The windows are still single glazed. A neighbouring flat is also rated D, with all the same features, except it is double glazed.
Does this sound dodgy? e.g. has he maybe paid someone a backhander to give the flat a favourable rating?