It’s been a very warm start to the first 13 days of September 2016 in the Central England Temperature [CET] daily series. Using provisional data it lies 10th warmest for the first 13 days in the daily mean series 1772 with a mean anomaly of +2.83°C, warmer than even the great September of 1959.
If you rank on daily maximum temperatures which dates back to 1878 rather than 1772, then 1959 is out at number one, with 2016 trailing by 2.26°C. The reason why September 1959 was so warm was that it was anticyclonic and very sunny.
Interestingly, September 1906 – the year with the highest September temperature – appears prominently in both tables.