The paroxysms that shake her body, in a corner of the King's Arms, Fenwick, where we've been having a pleasant lunch, remind me of Garrison Keillor's comment that the highlight of his childhood was making his brother laugh so hard that food came out his nose.
Susan is way too elegant and sophisticated to do something like that. But she is coming close to cracking a couple of ribs, if you ask me.
"Calm down," I tell her. "It wasn't that funny."
"It was," she says. "Go on, tell me again."
"I said my ex-wife is texting me a lot these days and you asked why."
"And you said maybe she wants to get back together," she says and laughs again, so loudly that other diners dart looks of disapproval and clouds of dust fall from the rafters.
"That's the funniest thing you've ever said," she tells me.
"I'm going to pay the bill then I'm leaving," I tell her.
"What was so hilarious?" I ask in the car, five minutes later.
"Your ex-wife wants to get back together?" she says. "You cannot be serious. You're forgetting that she and I are soul sisters. I've sat in her lovely living-room, making polite chit-chat and sipping her tea. I know how she feels about you."
"No you don't," I say. "Once a woman has known me she never gets over it. She yearns and pines. She haunts the night, beneath a waning moon, wailing for her demon-lover."
"Haunts the bars, beneath the neon signs, laughing like a hyena, more like," Susan says, and pats me patronisingly on the leg. "Take it from me son, she doesn't want to get back together."
"I have another theory then," I tell her. "She likes to keep tabs on me, using a mix of modern technology and witchcraft. She knows my every move. She went over to the dark side years ago."
"Don't be ridiculous," Susan says. "Have you got everything you need for the flight tomorrow?"
"Course I have," I say. "Been flying since long before you were born."
"There's been a few changes since then," she says. "Jets. Monoplanes. Airports."
"Look I know how to get from here to London. It's not hard."
"Humour me," she says. "Tickets?"
"Address you're going to?"
"Where is it?" she says.
"Back home, in the top drawer of my filing cabinet, in a black travel wallet you tuck inside your trousers, with a little loop your belt goes through, so you can never lose it or have it stolen."
She turns briefly to look at me, shakes her head, shifts down to fourth and takes the slip-road to Kilmarnock. "Who are you flying with?" she says.
"Flybe down, Easyjet back."
"Flybe takes various types of photographic ID. Easyjet are stricter, I think. It's not obvious from their website what they accept, apart from a passport."
She shakes her head again. "So much for a relaxing evening. You're going to have to drive back to your house and dig out your passport. You do know where it is?"
"Certainly. Of course. I think so. Maybe."
"Oh for heaven's sake," she says. "And your ex-wife wants to get all this back, does she?"
"Well no," I say. "This is what you might call the downside."
She pulls into her drive and stops the car. "I do," she says.
"There is an upside," I tell her.
"There is?" she says.
"There is," I say. "There is charm, wit, sex-appeal, suaveness, sophistication and intelligent conversation."
Her phone makes the new-text sound, so she lifts it and reads.
"Your ex-wife says 'No there isn't'," she tells me.