10th January 1982, when it dropped to -27°C. I lived in Banchory at the time, where it didn't fall below a balmy -24 °C. I remember the evening of 9th January 1982, as there was a splendid total lunar eclipse under crystal-clear skies.
The next morning, Sunday 10th January, I drove to Braemar and met the weather recorder (a schoolteacher, I seem to recall) who had taken the midday readings. It was -12°C
Here is a photo taken that day. (I am scanning old colour slides). The weather station has now been moved a short distance from the village centre.
Jack