Thank you. That's a lot of information.
I also know - or have heard - the following:
He was originally in the Royal Engineers.
At Dunkirk, he was the one who came up with the idea of driving lorries into the sea and placing boards over the top so men could embark directly onto the boats.
He was then posted to the War Office (while on a bus with his step-daughter who become my mother, an officious old lady poked him in the chest and called him a 'whitehall warror' until the conductor told her to look at his sleeve where he had "wound stripes")
He then went to India. I don't know what he did there.
After the war he went to New Zealand and became harbour-master of Christchurch or Aukland Harbour.
But that's all I know of him unfortunately. Because his name is so common - definitely 'Clarke' and I am pretty sure about 'Arthur' and 'Cecil' - I have not been able to learn anything more.
To view the images on office dump you need an key word which is 'officer'.