ATHENS (Reuters) - Former Greek premier Alexis Tsipras's leftwing Syriza party is leading the conservative opposition ahead of a national election next month but with a narrow margin, two new opinion polls showed on Saturday. The gap between Syriza and the conservative New Democracy party has shrunk to 1.5 percentage points according to a survey by pollster Alco for Sunday's proto Thema newspaper. The poll gave Syriza 22.6 percent against 21.1 percent for New Democracy, with 79 percent of respondents saying Tsipras had disappointed their expectations. ...