Hi mate, is the battery going flat whilst you are using the car or while the car is laid up? If it is going flat while you are driving i.e. the car breaks down so to speak then you are most probably looking at an alternator fault, basically it`s putting little or no charge into the battery so when you demand more from the electrical system like in the current weather needing headlights and heated rear screen on constantly the battery simply drains, once it gets below 11 volts the engine ecu cant function anymore so the car stops. If it`s a case of the battery drains over night then you more likely have a current drain, you need to get the car checked by an auto electrician to see what the current drain is at rest, it should be as close to zero as possible, most vauxhall`s draw approx 0.07 amps once all the systems have powered down, the most common cause of current drains is after market head units, the ignition and battery lives are the wrong way round on vauxhall wiring even though the plugs are identical to standard iso plugs, this has the effect of leaving the head unit on standby when the car is switched off and can drain a battery over night. Most of your local breakers should have an alternator for a Z22SE they are notorious for cam chain failure!