Derek's sister-in-law Jean is 67 and lives in a terrace in Crumlin. Last August she saw a mouse run along the skirting in the kitchen. Her husband Ray set a trap. They caught it.
"Problem solved," Jean says. "That's what we thought."
By the middle of September she was finding droppings every morning. In the cutlery drawer. Behind the toaster. Under the cushions of the couch.
By the end of September she'd stopped counting.
Then came the morning she rang Derek. Her grandson, who's 6, had stayed the night in the box room. She went in to wake him and found droppings on his pillow. Not near the bed. On the pillow. Where his face had been.
"I've never felt sick like that in my life," she says. "I stood in that doorway and I thought, what kind of granny lets a child sleep in this."
A week later the boy said something Jean took as good news. "Granny, I haven't seen a single mouse!"
Derek's face, when she told him, said otherwise.
"That's the worst thing you can hear in October," he says. "You're not seeing them because they're not coming out any more. They don't need to. They've moved into the wall. They've got a nest in the insulation and a larder in your kitchen. You don't see the ones that have settled in. You hear them, at 2 in the morning."