Ending a letter

[Unregistered]

I am writing a letter to a Sir, hence I am addressing it to Sir John Smith (for example). However, I believe the Salutation should be Dear Sir, so do I end the letter with Yours faithfully, or Yours sincerely?

[David L.]

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
eg

John Smith
Manager
Acme Chemicals,
SMELLY CITY

Dear Mr. Smith

cf
Acme Chemicals
Attn : The Manager
Toxic Waste Boulevard
SMELLY CITY

Dear Sir