USTA wrote:I want to write a complain letter to Home office.Can somebody advise on how to write a complain letter to me and where it should be sent?
Good luck. I wrote a complaint letter to the complaint department, and it took them 6 months to answer it

. So by all means write your complaint letter, but you should also take additional measures.
Someone mentioned that they have a "Customer Focus Group", which should be able to resolve issues quickly. I am not sure whether this replaces the complaint department or not. The address is given on the application form.
Other options are to complain to your MP, or to ask via SOLVIT about the state of your application. If you application is not completed in time, you should also file a formal complaint with the European Commission. I think this is also done via SOLVIT.
Tom