It seems an age since the ridge worked all the way to Kingsclere, but maybe the same rare conditions present on that freezing snowy day repeated themselves on Wednesday.
Here are the Larkhill sounding forecasts for 10:00 hrs and 14:00 hrs:
The gradient of the environmental lapse rate ended up inclining between the dry adiabatic lapse rate and the saturated adiabatic lapse rate, indicating that the air mass was ‘conditionally unstable’. So with the upward impetus of the ridge, the air on the valley floor, too cool to rise with its own buoyancy, became just sufficiently unstable to sustain flight.
See Tom Bradbury’s ‘Meteorology and Flight’ page 40 and http://www.ddsc.org.au/documents/weather/DDSC%20Meteorology%20Basics%20Rev%201.pdf page 50.