somethings obv draining the battery.
have you got anything plugged in the cigar lighter like a mobile charger ,things like that drain a battery if left long enough ,also radios on stand by .if you dont have any of these.Get it checked at a garage /auto spark, current drain up to 0.5 amps is acceptable any more than that will require further investigation,pulling fuses till the it drops.
disconnect the positive on the battery and connect a meter in line ie between the + battery post and the disconnected cable,when it settles it should read 0.5 amps if not pull fuses out until the current drops ,if you find it drops a lot the problem lies within that circuit.
Have you been to halfords for a free battery check, could be a duff battery