[W140Gill]

To my horror, i've noticed a huge split around the check strap (i think that's what it's called) on the inside of the driver's door. It must've sneaked up on me! Is this a common problem with these heavy doors?

What is involved in door removal? Is the wiring conduit easy to disconnect etc? Any info on the procedure would be welcomed, as I'd like to weld it myself!

not a common problem. Sounds like your door has been over opened at some stage and then the crack has grown with usage. You will need to get it welded.
See my post on removing the door card and take it from there