I found them !
Whilst sitting on a hillside in Anho in Oxfordshire with Jessie the dog, we watched a huge storm roll in. Although I took this series of photographs at the time, they didn't show the energy of the storm.
Having painted this sketch in watercolour I realized the only way to share and evoke the mood of the immense power is to paint a six foot version on canvas.
After rediscovering the 20 year old photos I have a large canvas primed and
ready to go .....