starting problem -1999(v) vauxhall combo ?

hello,
van - 1999(v) vauxhall combo 1.7 diesel merit
starting problem- my mate said its the glowplugs !
so i replaced the glowplugs(diy) and the problem is still
there! when i turn the ignition key to start engine
it er er er few times then starts up on one key turn !
er means the sound starter motor makes ok!
and when i try to start it again later,the next morning then the battery goes flat ! please can you tell me
whats the problem !! ?? THANKS !! stressed !!