Even more history
Waltham actually opened as a civil airfield (Grimsby Airport) in about 1934. My father trained to fly there in 1938 under the Civil Air Guard scheme, although when war broke out and he applied to join the RAF they turned him down because "we have enough pilots at the moment thank you". When they came back to him in 1940 it was too late as he was by then wearing khaki!
(As another aside, when I went solo at Biggin Hill in 1966 I invited him into the clubhouse for a celebratory pint, and he immediately recognised my instructor - the same one who had sent him solo in 1938!)