You would not start the letter Dear Sir, but Dear Sir John, as he is titled. This is not being over-familiar, but merely the correct form of addressing someone with a knighthood.
You are writing to someone whose name you know and use in the letter. Therefore, you sign it, Yours sincerely.
If you ever know the name of the person you are writing to, do not use Dear Sir
Dear Mr. Smith
Attn : The Manager
Toxic Waste Boulevard