4° here in south Manchester earlier this morning. Don't think the forecast was anywhere near that last time I looked. I had been expecting about 11°. Minimum temp can change a lot in a couple of days, from the forecast to actual. This will be due to clear skies, wind direction, or a cold front.
Ran the heat a bit yesterday to lower the humidity (had the windows open as well, but not at the same time), and got radiator hammer in the bathroom after a while. Tweaked the TRV to stop that. It's ironic really, and counter-intuitive, but when it's cold outside, and the weather apps say humidity (they always quote relative humidity) is very high, say 98%, the
absolute humidity will actually be a lot lower than it is inside your house, so you should open the windows to lower the humidity inside. Do this to prevent mould.
You can calculate the AH here.
https://planetcalc.com/2167/#google_vignette
Understanding this is actually simple. Cold air can't hold much moisture. Hot air can hold a lot.