The floor on the drivers side of my 2009 Astra is absolutely soaking wet

Some of what you're describing I'm having a hard time understanding, but to start with, there's two common things, other than leaking window seals, to look for. The first is if you have a sun roof, there are four drain tubes with hoses running to the bottom of the car. If one is plugged or blocked with debris, you'd think water would flow through the others, but if the car is leaning just a little, the water will spill over into the car before it gets high enough to reach those other hoses. The clue is something around the sun roof will be wet.

The second thing to look at is if a repair was done inside any door, there's a plastic sheet glued to it to keep water from getting inside the car. When it's removed, it is usually stuck on so tightly that it stretches, then when it gets put back in place, gaps form that can let water leak inside the car. To identify this as the cause, leave the doors open a couple of inches and place masking tape along the top gap to keep rain out. If the car still fills up with water, it's not a door issue. Look for water running down the inside of the firewall or behind the back seat.

Also look at the fresh air intake in front of the windshield or under the hood. If that fills with leaves and the drain hole gets blocked, water can build up high enough to run inside through the heater box. I've never run into this myself, but I've read about it a couple of times.