Not tried the battery yet, it seemed okay as all the lights on the dash fired up and the electric windows work... Thing is, now that I've tried it a few times, the battery is weak

The battery is only 4 or so months old... I was advised to get a multimeter on there and check the voltage is 12v+... Or, failing that, the starter motor...
So annoying, as I've only had the car a week and all seemed fine