Gently Benevolent: Like all children outside France, I was not born evil.
Parliament gets roasted, both in the figurative and the surprisingly literal sense.
Crosses the Line Twice: The Chinese Restaurant scene from the second season, as well as Mr. Benevolent's Fagin imitation.
St. Bastard's wouldn't be half as funny if it were any less awful.
Draco in Leather Pants: Well... in the fourth series, Gently Benevolent starts wearing leather pants, and by some strange coincidence this is also the series that calls most attention to how good-looking he is and where women seem to be falling for him all the time.
Taken even further in the appendix to the novelization, where it's revealed that at the age of 90, long after the death of Ripely, Sir Philip and Mister Benevolent (well over a hundred, but thanks to dabbling in the dark arts still appearing in his forties) set up a home together in Worcestershire. For the next twenty years or so they're rumored to be traveling the pan-dimensional universe together — though, the text implies, they "may just have stayed in Worcestershire, really, really quietly, without anyone noticing."
Tearjerker: Poppy Bin's death. Made even worse when the Hardthrasher of the week claims she's probably in Hell, despite claiming just seconds ago she'd be going to Heaven, just to be a dick. Heck, even Sir Phillip Bin admits he's still broken up about it, decades later.
They Wasted a Perfectly Good Character: Sweetly Benevolent née Delightful is killed off-screen at the beginning of series four. She would've been a fun new villain in counterpoint to the (at that time) reluctantly evil Gently Benevolent, or as a Dark Mistress when Benevolent was inevitably re-eviled. Instead, the role is given to Pippa Bin.