Largely unannounced, off F&M’s gent’s department, this bijou bar is a pretty prospect – despite certain furnishings suggesting a fusty old St James’s members’ club. The backstory behind the name ‘3 (shillings) and 6 (pence)’ refers to the amount 1930s customers paid per head to have Fortnum’s throw a cocktail party at their houses (booze was charged extra). From a limited list of familiar period-piece Italian and American mixes (the likes of Negroni, Tom Collins and Champagne Cocktail), a No.3 Dry Martini is unimpeachable. Some London classics would be welcome, however, and it’s frustrating that off-menu requests aren’t accommodated. Moreover, Tequila, scotch, brandy and rum-based cocktails are all absent. Bar food is fair – steak tartare, tuna sashimi, a fresh but underpowered panzanella, a Henry VIII-sized portion of devilled kidneys on toast – but tweaks may be needed if this new spot at normally sure-footed Fortnum’s is to take off.