Properly speaking the Percy Special appears to be associated with the English/Scottish Borders so Yorkshire (Sergeant Cluff's home) would be at the southern most edge of its reach, but people need to be warned, and the thing is in the spirit of the books setting.
Written in the late 1950's/early 1960's the two Cluff books I've read are best described as Yorkshire noir (original write up Here). They're bleak, brutal, and very British in the 'Get Carter' mold. From memory Cluff mostly drinks beer, or brandy, but as the brother of a well enough to do farmer in a county where there's both hunting and shooting a Percy Special is a distinct possibility. It's the kind of no nonsense, strong, drink that you could expect any farmhouse kitchen to be able to rustle up.
It's a concoction attributed to the 10th Duke of Northumberland (1914 -1988), and quite possibly invented with malice aforethought, the Percy Special is simply equal parts cherry brandy and whisky. I first encountered it one new year in the Scottish Borders, it came in a fairly large, full, tumbler, and despite protests was followed by another. It pretty much did for me.
It's very much associated with hunting and shooting both of which tend to start with strong spirits, and it's a good hip flask option for cold days on the side of a hill. Or inside after a day out in the cold. It'll certainly warm you up. It is not sensible to drink it by the tumblerfull, and beware old lady's who tell you otherwise. Nevertheless it has become something of a New Years tradition to drink this at least once, and in moderation I like it.