The Doctor turns his head, glancing off along the street where they'd disappeared.

"Costello," he repeats. It rings a bell. Money laundering, undercover drugs ring, something like that. Maybe a bit of both. Maybe he read that somewhere.

Her suggestion brings him looking back towards her once more, and there's a slight smirk at her very suggestion.

"Yeah, don't tend to be very good at that as it happens."