Battery Drainage & Immobiliser issues (Ecotec SE8)

[SE8 Rob]

I tried to find a topic similar, but couldnt really find anything the same.

The other night I was using my car for work, and it was totally fine... I left the car outside for 10 mins and when I came back to start it there was issues with it trying to tick over. I turned the ignition off, and my battery light was on. It was peeing down with rain and pitch black by now, so I just went home. The next day I went back, attempted to start it and the battery was completely flat!

So after a trip to halfords (and feeling slightly ripped off by them, but having no choice but to go there being a sunday) I get a brand new battery. I fit it to the car, try to start it and the engine checklight flashes, which as far as im aware is an Immobiliser fault? I do as the manual says (which is effing ridiculous) and finally get it started. I assumed this would be the end of the problem and take the car home, the next day, the battery is flat again and it keeps giving me the same bloody Immobiliser fault even when there is a fully charged battery :s

Any ideas? lol

[SE8 Rob]

Because with the new battery it wasnt starting, had to check what the flashing light was, and the manual basiclly said "Turn key to O position, wait 2 seconds, then attempt to start, if this fails, try again" :s

I thought it might be the alt, but would that drain the battery whilst it was just left in a carpark?