Ok, sorry guys but I have not seen a good explination of the M thing, except just ignore it. Yep typical there.
A friend of mine have talked about this, and came up with a good idea in my opinion.
Yes there have been multiple Bonds, but none of the Bonds realize this, they where taken in and brainwashed to believe they are Bond. This explains why M is still the same even though you should have a male M in Casino Royale.
I am sure you will all attack me for this idea, but I challenge you to explain why it is a bad one instead of just saying "it sucks".