London - Sainsbury's supermarkets remained on high alert today amid fears that the so-called Mardi Gras blackmail bomber has switched his attacks from Barclays Bank. Incendiary devices detonated at stores in west London on Saturday and one man was injured. The bomber warned in July last year that he would attack Sainsbury's unless he was paid a reported £500,000. But the bank is also keeping up its guard in case he renews the campaign which saw him target Barclays more than 20 times between December 1994 and April 1996.