Here in Edinburgh, last night's overnight minimum temperature was -1.7°C at Edinburgh Gogarbank and around -3°C at Edinburgh Airport.
I have been looking back at the records going all the way to the year 2000 and up until last night, the lowest overnight minimum temperature which I could see as being recorded during September was 0.7°C as recorded on 27 September 2007 at Edinburgh Gogarbank.
Amongst all of that data, I have not been able to find any other occasion apart from last night, where an official air frost has been recorded here in Edinburgh during the month of September (if anyone else can find such an occasion, I will be more than happy to hear about that on this thread).
In all of the years that I have lived, I can remember there being a lot of frosty mornings during the autumn, but I don't remember any these occurring any earlier than October.
Furthermore, Edinburgh Gogarbank is only a relatively new station which has only been around since around 2005 so when all of the above is taken into consideration, last night is looking likely to have been the coldest September night on record at that particular station at least.
The north of Edinburgh, usually always missing out on snow events which occur not just within the rest of Scotland or the UK, but also within the rest of Edinburgh.