You need to get a voltmeter on it to check, or alternatively take it out and go to Halfords with someone and they can check it for free, although they will try to sell you a Halfords battery and they are £££.
The battery might not be knackered as such, just drained. At this time of year, you've got so much electrical stuff on, that the drive to and from work can sometimes not be enough to keep them fully charged.