Here's Betjeman's description of Sir Horace, from a letter dated 10 Februry 1929:
I am at the moment Private Secretary to Sir Horace Plunkett
who in the early eighties was a big man in Agricultural Co-
operation. He is still more than keen on it and being slightly
off his head has written the first chapter of a book of nine
chapters no less than seventy-two times. He says the same
thing over and over again and rarely completes one of his
sentences which suits my style of thinking. The pay is fair
and the food and travelling excellent. He is in bad health
at the moment and this hotel** is furnished in that
Japanese style so popular with the wives of Anglo-
Indian Colonels who retire to Camberley . . . (p.52)
Needless to say, Sir Horace was not 'off his head', just clearly suffering from a bad case of writer's block. I've heard him described in all seriousness as one of the great men of his century for his devotion to improving the lot of farmers, particularly in Ireland through the Co-Operative movement. It says a lot about his character that when private airplanes came in when he was already an old man he had someone take him up so he cd better see for himself the patchwork of fields and farms and how they all fit together. As a result, he learned to fly himself when already well into his seventies.
*this was back in the days, only a decade after the Spanish Lady, when folks took flu seriously.
**the Beresford, in Birchington-on-Sea, Kent