This was my post.
"DavidK, regarding the Thom Aubrey measurement. 897.6 feet circumference,285.7 feet dia, is the size Thom went with when he originally used the Ministry of works plan. After he made his own survey he found this to be wrong and changed it to 283.6 feet dia, which is 890.95 feet circ. This would be a mean dia. for the hole centres. 285.7 feet would fall within a kind of ring composed of the mean dia. plus/minus an average hole dia. I have found his 283.6 feet to be a very good fit as an average."