Hi,
I also have a 2005 1.7 CTDI astra.
The battery also has a health indicator on the top of it (Should be green transarent circle - if cells are ok).
Do the interior lights come on when you unlock the car? These may also suggest the battery is ok.
I currently have a problem with my car starting - I have charged the battery, the volts are good (@14 V) and the fan belt/alternator are ok.
It seems as though I have a starting motor fault - sometimes it turns the engine over, and other times it doesn't I have read of similar issues on the net. Or their could be a drain on the battery somewhere , eg an interior light left on, radio on, a glow plug failure, or other. The garage should be able to check if the draw current is abnormal.
No solid answer, but should help you investigate the problem! ;-)
Marcus, September 2008