as rally says if your doing lots of short journeys especially at this time of they year with the heater , heated rear screen, lights on etc its going to hammer the battery coupled with the harder to start engine due to the cold
if you have a drive way bang it on charge when you get home
i have a lidl version of this
http://www.argos.co.uk/static/Produ...ment+and+accessories+/+car+battery+chargers+/
which cost me about £12 check your local lidl and aldi they might have some left
i cut the crocodile clips off and soldered a cigarette lighter plug to mine,
Assuming your lighter socket stays live when the car is off just plug it in the fag lighter and set it to car mode and leave it saves opening the bonnet every time simple way to check does your fag lighter get hot when the keys are out of the ignition?
(i have installed a extra fag lighter socket in my GP so that one is constantly live)
how old is the battery on your car at the moment?
Most places will do a free battery test but don't beleve the first one and get quotes, as prices can vary dramatically the bit high street national chain starting with a H is usually very expensive.....
also make sure you have your Key code for the radio if it is coded before removing the battery