After a trip to the scrappy today, I've had a fair bit more light shed on this subject. There was only an old D8 down there, but I'd assume that the mechanisms are all the same. Please excuse the photo quality, but I don't have a super-duper touchscreen £500 phone.
Got into the back & took the rear door card off & peeled back a good chunk of the membrane (looks impossible to remove so that it can be re-fixed) to give me access to the mechanism. To take the exterior handle out of the door, you need to undo 2 nuts. One is located inside the door:
Have you ever wondered what was behind the little blanking plug at the edge of the inside of the door? That would be the second nut:
Once the handle was out, I had a good clear view of what needed doing, although to be honest, now I know what needs to be done, it could probably be easily done with the handle in situ. See the small white clip in the picture:
Before I removed the handle, this was where the rod was fixed in that links the handle to the door lock. Note that in the picture above the clip is at the bottom of the slot. That bit of movement before the handle opens the door is where the clip needs to be at the top of the slot to start working the door lock.
So, to solve the problem, we need a way to make sure that the white clip is always at the top of the slot. Lucky enough, somebody left some cigarette filter tips in the back seat, so I made use of one:
Seems a bit of a bodge, I know, but when I put the bits back together, the handle opened the door with no slack. This wouldn't be any good for a permanent fix as the filter tip is too squishy & could fall out, however the right sizedsmall nut & bolt fixed in the gap would solve the problem on a more permanent basis.
Next time I get a couple of hours spare I'll try it on my doors & report back.
I guess the slot is either so the clip can be passed through it & twisted and/or the lever isn't tugging on the release all the time - with the possibility that the door could pop open...