I am even later to this wonderful conversation, however, I joined mainly in case I could add anything of interest regarding the 'Chocolate Plane' of 1930.
My grandfather was responsible for this plane - George Sara. I have a letter from E.E. Barringer (Sweet Success author) responding to my cousin in 1980 with some snippets of information and supposedly copies of the 3 photos Cadbury had of this plane. Sadly I only seem to have one, it is a larger copy of the one used of the plane by itself in Sweet Success. There were some good ones blown up quite large, down in Dunedin at Cadbury World. I don't know if they are still there.
Barringer references a Centennial booklet in which Sir Carl Smith wrote as follows:
"The Chocolate Plane was piloted by a New Zealander, Captain MacGregor, who, as a pilot in the R.F.C. in the First World War, earned himself the name of 'Mad Mac' on account of his flying exploits. Apart from flying executives when required, the plane visited all the centres and took customers for flights - flying for civilians in those days still being a novelty. Unfortunately, after leaving C.F.H [Cadbury Fry Hudson], Captain MacGregor was killed in a flying accident."
I see in my father's photo album, a photo of him aged 10 or 11 in 1930, dressed in flying gear, with a label 'Dunedin'. So presumably he was one of the lucky ones who got a ride in the plane. I can barely imagine what kind of story that would be for his school mates!
My mother used to tell with great glee about her own ride in a Tiger Moth, but that was not till the late 1940's when she worked for a Timaru travel agent. It clearly wasn't a common occurrence to be a passenger in one, even that much later.
As to why Cadburys soon stopped using the first Chocolate Plane - I can only speculate, but my grandfather left Cadburys in 1931. He had been Manager since 1926, and sent out to NZ right as WWI started - then returned after, to work for Cadburys. His father (also George Sara) was a highly successful travelling salesman who passed on a child's suggestion that the new recipe chocolate coming out in 1905 be called 'Dairy Milk'. His funeral in 1917 was attended personally by a member of the Cadbury family who then went on to visit NZ in 1930 at the time Cadbury amalgamated with the Hudson brothers.
My grandfather went from being the sole manager of Cadburys in NZ, to being the only 'Cadbury' director alongside the SIX Hudson brothers. Hudsons did many things quite differently, and in particular, were not great spenders on advertising. One can only wonder what they thought of the ludicrous expense of a company plane... the official story is that my grandfather 'retired' - but he was in his 40's with four dependent boys and two step-children from his second wife (his first wife died in 1927). The book 'Purple Reign' describes the amalgamation in 1930 as 'sticky'. For a man who'd been in WW1 himself on the Western Front with the NZ Field Ambulance, newly married, then tragically a widower just 8 years after the war ended with four boys under the age of 10... who had been used to such independence... the changeover was clearly just too much. He went to work in Wellington for a government department.
I have copies of correspondence from the Mr Barrow Cadbury who visited NZ in 1930, sadly he doesn't mention the plane in those.
Sorry that is more personal history than about the plane itself, but I hope it gives some interesting background.
Robyn