Vectra starting issues.

[GEARJAMMER]

Ive got an 08 reg Vectra 1.8 SRi, my problem is starting it, sometimes i put the key in, turn it and nothing, dead, nothing! Other times it just clicks like the battery is dead, others times it will turn over but not fire and other times it will start first time, it really is quite random.

Any ideas what the issue might be (other than the fact its a Vectra).

Thanks in advance.