Have you spoken to your agent? Our agent was in constant communication with our CO and updating her with any information, such as when our medicals were done and then finalised. Because of our agent was on the ball our visas were granted yesterday, 25/03/2014, the same day our medicals were finalised.
I would seriously recommend getting your agent to contact your CO and ask about the progress. This will highlight your application and get you your grant letter.