The cause of this evening's spike in particulate matter levels would appear to be the fire near Lydd in Kent, 44km (27 miles) away. Here in the Sovereign Harbour area of Eastbourne PM2.5 peaked at just over 240μg/m³, while PM10 peaked at just over 260μg/m³.
We are in the middle of a heat wave, with unusual wind patterns (eg. wind out of the east), which is bringing us the smoke. The Eastbourne Herald has a report on the Lydd Ranges fire.
Despite the huge peak, because levels were only high for a short period of time, 24h average values are presently at about 23μg/m³ for PM2.5, slightly higher for PM10. These values are comfortably within the DAQI "Low" range (0 - 35μg/m³ for PM2.5).