NATIONAL POWER CORPORATION, PETITIONER, V. HEIRS OF SALVADOR SERRA SERRA, HEIRS OF GREGORIO SERRA SERRA, MARGARITA SERRA SERRA, FRANCISCA TERESA SERRA SERRA, FRANCISCO JOSE SERRA SERRA, SPOUSES PRIMITIVO HERNAEZ AND PAZ BACOL, SPOUSES BERNARDINO MONCERA AND ROGACIANA HERNAEZ, SPOUSES AMBROSIO FORTALIZA AND LUISA HERNAEZ; BANK OF THE PHILIPPINE ISLANDS, REPRESENTED BY ITS MANAGER, LUIS A. PUENTEVELLA AND ARSENIO AL ACUÑA, RESPONDENTS