Oh, is he? John sneers in reply to that, because he supposes Ianto is trying to be funny to deflect what he's done but it isn't working.

"Fuck you," he says, and although he's still simmering it's as much tired as anything. "I know what my limits are, Ianto. I also know yours are a lot lower, but if you want to make yourself faint be my guest and I'll just finish fixing this myself."