[SAJ]

Hello Rob. Over the years the rear door post sags outward. A crack can appear where the level changes near the roof bow pivot too, making the outward sag of the door latch post worse. If you lean on this rear door post to push it inwards the gap at the door bottom will decrease.
Marco Tahtaras posted a good explanation somewhere.
I followed this, inserted a reinforcing piece between the several steel layers where the crack had formed, pulled the post in with a ratchet strap fastened scross the car and spot welded the reinforcement in to avoid burning too much good paint. The repair is invisible and my drivers door shuts nicely with a rigid click, since the rear door post flexes much less now.
I hope you can follow this. I am posting from my Samsung Tablet which is not very easy.
I may be able to find some pics if this would help.
SAJ in NZ