Elevated PM2.5 levels stayed steady yesterday daytime before beginning a further increase in the evening from around 40ug/m³ at 18:00 GMT to a peak of over 115ug/m³ at 20:45 GMT. This is not good. The floating 24h average value is now at 55ug/m³ and still rising. This is very unusual.
At 08:55 this morning PM2.5 is at 41ug/m³.

Causes?

Slow winds - avg 1mph, gusting 4mph, from SW (ie. not the sewage works, though low wind speed means that pollution is not blown away so fast).

Low temperatures - day max 8.1°C, night min 1.3°C. This would lead to an increase in fuel burned for heating. 21:00 is a reasonable time to expect to see a cut off if that is the cause of the pollution. Although, everything else being equal I would have expected to see a corresponding increase in pollution this morning as boilers were switch back on and people began putting wood into their burners again.