I think this story was just asked for an another forum, with a slightly different description, but I am pretty sure it is the same one. This is towards the end and the retired or just extremely elderly Pemberley vicar shows up with the family bible because Miss B was purposefully found in Darcy's bed that morning and the Earls were all trying to figure out whom Darcy would have to marry.

A Reputation is an Easy Thing to Lose by Stormsmith over on archive.