The Oxford Street Coffee House

Elsie Jones, John "Jack" Young, Mary Moore and George McDougal all live in service to a secret:
Scotland Yard's best source of clues and leads is actually one of London's oldest coffee houses.
These four young people manage the house as waitress, clerk, manager and owner-but they moonlight as England's finest undercover detectives.Therefore, when a priceless portrait from a traveling Da Vinci collection goes missing, they are the first on the job. Using their unusual set of sleuthing skills, and with the unknowing help of their tale-telling customers, they must track down the thief
before he makes off with another great work, without betraying their true identities to the patrons of the Oxford Street Coffee House.