Ken, as Andy says the focal length is 600mm, that is a 300mm + 2x.
Andy, the crop is not that big. I was too close to get the whole bird in the frame whilst still trying to keep AF somewhere in the right area.
Really I should have switched from AI servo to one shot and used focus lock, or better still, I should have taken the 2X off.
Thanks for all comments and best regards, Mike.