If you already had your biometric info taken in the UK you won't need to do it again (as long as you have provided the BRM details on the form).
Also HO can take a long time, depending on what your circumstances are and in what stage your application is. When did you send off your application, did you use NCS?
Otherwise you can contact HO with regards to Citizenship
here
Also, do you have a source which states that the letter is only valid for 15 days after which the application is refused? I have never heard of this before, nor seen it in any of HO published documents.