The bottom of the pocket section behind the door pillars is quite a way (30mm ish) below the drain hole, the bottom is the same level as the top of the sill inside the car. So the pocket sits there full of water, not a real problem except the sealing around the pocket area is not good on most 2+2's and the water leaks out, normally along the top of the sill a bit and out over the sill next to the door opening.

My fix is to thoroughly clean out and dry the pocket area (it will have several mm of silt collected over the years), jack the car up at the rear and then pour potting compound into the pocket until it runs out of the drain hole. Let it run out wiping it off (you did pour it in slowly didn't you) until it almost stops, then stick a bit of tape over the hole. When it's set clean out the drain hole with a drill. This fills the pocket to the level of the drain hole and the car being jacked up at the rear while it sets means it slopes towards the hole when the car is level. It's better to use a compound that stays flexible rather than epoxy or fibreglass resin which might crack away from the sides and let the water leak out again.

I used polyurethane resin, obtainable from RS you need quite a bit, the pocket is fairly big, you could try filling it partly with expanding foam first but it's tricky stuff to predict.