I'm learning too Keith. Those planes are exquisite.
You started with the background, absolutely correct, but I think it's a good idea to make it very soft, light and splodgy because the fine detail is destined for the most important part of the painting, in this case, the planes . Standing back the strong colours of the clouds jump out and that brilliant detail of the planes is a bit lost. Similarly the cliffs are very organised, no rockfalls here or variety of sizing of rocks to break up the neatness. Someone once told me to smudge a bit of earth on the least important bits!
You can always brighten up the landscape a bit at the end making sure that it is not likely to overpower the main centre of interest.
Haha! Laughing at myself now! After seeing a David Hockney Exhibition recently, I've been exploring the idea of creating interest on every bit of the canvas. It still needs the strong and calm places though. He seems to do it but I can't. It will take years to achieve that!