Overnight PM2.5 levels continued to climb, passing 40μg/m³ at about 2300hrs, and oscillating up to 50μg/m³ at points, with brief periods of lower pollution as clearer air from the sea eddied in to the area.
The pollution source appears to be UK-based, with MetClim showing little in the way of continental pollution heading our way. This tallies with the weather station readings showing very low winds mostly from the north through north-north-east.
As I type, the 24h average PM2.5 level, which DEFRA uses to calculate is DAQI value, is 29.0μg/m³ and rising. This is rated using DAQI as index 3, in their "Low" range. Moderate begins at 36μg/m³.