Hi Rod, are you sure there was an original horn - of any brand? There's no mention or picture of a horn or horn button in the owners guide. Probably due to the new fangled electricity being too complicated.
Sparton is an US company and would certainly have been period, but Lucas was making Altette horns in 1930.
Unless it was a hand operated klaxon type, I would wonder why they didn't use the local brand?
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.