Mycroft does not look pleased. He positively glowers at Guppy before snapping his fingers. Predictably enough (to himself, and possibly Sherlock), Work Experience waitrat makes himself present quite quickly.

"My brother lost a tooth behind the bar," he tells the rat. "Return it to us."

He hands the rat a £10 for its troubles. What the creatures do with his money, he's not sure, but it has earned him the most amazing loyalty.