The coldest September in the UK on record, well in the abridged 1910 temperature series from the Met Office that is, was that of 1952, with a mean anomaly of -2.79°C (fig 1), in fact the headline in the Monthly Weather Report for the month read – Unusually cold.
It was also the fourth coldest September in the CET series that started in 1659, and still holds three extreme minimum daily CET records from 1878, and also four extreme low maximum records as well (fig 2).
September 1952 was generally anticyclonic from the 7th to the 16th, which did allow some night frosts to occur, before turning very cyclonic from the 25th (fig 3). You may have noticed that September 1986 was the joint third coldest in the UK since 1910 (fig 1), but unlike 1952 this was mainly due to the anticyclonic nature of the weather (fig 4), in fact 1986 is the joint 10th most anticyclonic on record since 1871 in the Objective LWT series. So September 1952 was cold both by night and day, whilst 1986 was colder at night than day. In contrast to either of those two years, 2017 has so far turned out so far to be generally colder by day than by night.