Jayand,
Reference your post 55....
First of all PingDit got it wrong, it was CAS, not COS, that flew in an MRA4. As to when, and proof.....
The July 2010 edition of Frontline, a BAE Systems glossy newspaper, has, on its centre pages, a picture of CAS in a flying suit, with a Nimrod MRA4 (with an "A" on its nose) in the background. The caption says that CAS had just flown in "....the first production MRA4.....".
I have no reason to doubt the veracity of this article. As it is in the July addition, and given lead times for articles generally, I would surmise that the flight took place somewhen in the first half of the year.