It is A Jolly Kind of Detective Game by custardpringle, posted at AO3 and weighing in at a whopping +139,000 words. But size isn't everything. It also has mystery, adventure, red herrings and romance as Hilary Thorpe and the Viscount Saint-George come down from Oxford and find themselve plunged into rather more excitement, local journalism, and organised crime than they had bargained for.
It is a pretty shippy ship-fic, the pairing being (as you might expect from the protagonists) Hilary/Saint George, so if you run from that sort of thing as from the plague it may not be for you (it certainly has the highest rating of any Wimsey het-fic on the internet). What more can I say? It's a Wimsey fanfic novel and it works as a novel, and if you like Sayers, or Golden Age detective novels (elements of it reminded me of Margery Allingham, which I don't think is an accident on the part of the author), or faux GA novels, take a look!