2004 Vauxhall Astra Leak behind driver's seat

Hi there

My Astra has a leak and is very wet behind the driver's seat. It happens when the car is parked and there is heavy rain, and there is no water anywhere else apart from on the floor. Every part surrounding it is bone dry and I cannot see where it could be coming from apart from up from the floor, but I cannot locate the source. Any ideas what to check first?

Thanks