There’s a very good gag in Mick Herron’s new “Slough House” novel, an actual laugh-aloud moment — but only if you’ve watched The Great British Bake Off and, more specifically, the sponsor adverts that used to surround it. If you haven’t, I think it comes across as a moment of silliness.

You can’t unknow what you know, so I’ll never be sure, but I think that silliness works. You have no doubt, given the context, that it is a joke, but it’s also delivered in a moment that is otherwise acutely serious.

That’s always so difficult. A tense moment can be the perfect point for a joke and I do believe that you need serious and humorous, that you need light and shade. But so often a tense moment can be punctured because of a funny line. Or more often, a line that is intended to be funny but fails. You know the kind of thing: the hero is faced with a firing squad and says with a raised eyebrow, “Ten soldiers? I thought there was a manpower crisis.”

Jokes like that are not there to be funny, they are there to impress us with the hero’s bravery and I think it’s fair to say that without one single exception, they do not work. They cheapen the drama, they lower the stakes.

So here’s Herron and if you get the joke, he is running a gag dead centre of a serious point. And if you don’t get it, you see there’s some silliness — dead centre of a serious point.

That, I think, is some marvellous writing. Let me point you at it: “Clown Town” by Mick Herron, the latest in the series better known as “Slow Horses”.

Only, I’m surprised I like it so much since, as I say, more often the joke is this cheapening type. As it happens, this one is a reference, and I’ve wondered whether reference jokes are dangerous. I’ve thought before that referring to something outside of the fiction breaks the story. It tells us that the story is just a story, that it is one tale amongst many others.

In the same book, Herron makes it clear that a character is listening to the theme song from the “Slow Horses” TV show and that one feels contrived. It isn’t the type of music you’d imagine that character listening to, for one thing, and overall it seems like the writer nudging you in the ribs.

It does take you out of the story and to me that’s unforgivable. It’s so hard to get someone into your story, but it’s harder still to get them back in after you’ve chosen to thrown them out.

Yet here’s this reference to a TV show that I didn’t think worked, and there’s that reference to the adverts around the Bake Off, which I think does.

Let me go check my Rules of Writing book, I’ll get back to you.