A far better article than the one in the Telegraph Pippi - thank you for posting it.
In Bucks it looks like there is a small "bias" in favour of older children - perhaps that could come from the appeals system?
I think this is unlikely, given the number of successful appeal cases each year. They wouldn't account for the difference county-wide.
Bucks appeal panels do recognise the disadvantage of a late birthday and the relative immaturity that accompanies it, and if anything, I would expect the success rate of appeals for late birthdays to be slightly higher.
In Bucks I think the bias towards older children has more to do with the youngest children starting school two terms later, and thus having less incremental learning by the time of the 11+. I am glad that many schools are now abandoning the idea.
Nationally the picture will definitely be skewed by all the areas where the 11+ is not age-standardised.