These photographs are the latest in a long-running series of office workers in the streets of London's financial district and around the docklands development of Canary Wharf. What began 30 years ago as an exploration of these overbearing buildings has distilled to a close sociological study of their day-time inhabitants. Themes of estrangement and dislocation emerge; the spaces between members of this peculiar tribe are as significant as the figures themselves.

Young, smartly dressed women feature prominently, striding between work-station and sandwich-bar, with men in pursuit as if directed by Hitchcock. Men in blinding white shirts, hands thrust in trouser pockets, parade like stock figures by Magritte; stairs and concrete decks convey regularly spaced figures as if on an eternal treadmill designed by Escher. Headphones and electronic devices induce a state of reverie to their self-tagged recipients. There are precise distinctions between the gestures and stances of each sex. When the air is chilly, women hurry with their arms crossed; when waiting for a date, women stand with legs crossed at the ankles. Men never do.

The medium of black & white film suits the serious, sombre quality of these streets and buildings, unruffled by the frippery of shops or tourist attractions. The significance of the smallest human gesture would be lost among the distractions of colour. Sack says: 'With digital, the temptation might be to keep a running check on the captures. I enjoy not knowing what has been nailed until weeks or even months later, when I develop the films and — if I'm lucky — discover an unimagined conjunction or a happy accident.'