Water leak Vauxhaull Corsa Design 2008?

Hi. I have a 2008 Corsa Design and I have a leak in the passenger door - the end nearest the glove compartment. If it's been raining heavily the night before, it's like a water fall pouring in to the passenger footwell when I drive off. The seals all look fine and I can't see where it might be coming from. I've stuffed paper towel in under the door and this seems to soak most of it up - but with winter coming on, I'd really rather get this sorted. Any suggestions?