Frightened women sank to their knees in pray as darkness rolled across the skies of London at lunchtime yesterday, lunging day into night. Crowds in Piccadilly Circus seen here groped their way through the blackness and motorists crawled slowly along the city's dimly-lit streets as a great blanket of smog crept form North to South. At 1.25 p.m. the vest End was wrap2ed in a light grey fog. At 1.30 p.m. it was as black as midnight. Seven minutes later the central area was clear again, but the rol