Tesco has extended its lead over arch-rival Sainsbury in the battle of the supermarkets, figures out yesterday showed. Tesco's share of the British grocery market grew by 0.5 percentage points in the second quarter of 1998 to stand at 24.7 per cent, 4.2 per cent ahead of Sainsbury, which slipped back to 20.5 per cent from 21.3 per cent.