2006 Vauxhall Combo Will not start

HI, I have Combo diesel 1.3. A few weeks back the alternator light came on which i ignored for 2 days until the car finally would not start. RAC came out and got it started just enough for me to get home. I bough a new alternator (well secondhand). It was cranking like it wanted to started but battery began to fade quick, I put some more fuel in as light said it was empty - still would not start. So I bought a second battery that was 40 amps less than one in there - still would not start. Put old battery back and charged it fully. tried some easy start - still wont start. I've just replaced 3 glow plugs as it as only using 2 before, but sill won't start, although the easy start got it go for a couple of seconds, I guess until it had burn that gas off. THe battery does seem to go flat quite quick, enough to stop t cranking but enough to use the lights etc, goes to about 8 volts. So any ideas? maybe buy a new battery?

Thanks for your help