UPDATE 1-Tesco, Asda sales fall as discounters advance-Kantar

(Adds details, table) LONDON, Aug 25 (Reuters) - Britain's "big four" supermarkets have all lost market share in a barely growing grocery market, with Tesco and its main rivals failing to keep pace with discounters Aldi and Lidl, Kantar Worldpanel said on Tuesday. German-owned Aldi and Lidl now have a combined market share of 9.7 percent, versus 8.4 percent a year ago, according to the market research company. Like-for-like sales at market leader Tesco fell 0.9 percent in the 12 w