I have started using the DJI FPV hand controller, love it, but I can't select M mode, only N & S. Anyone got any idea why?

Pretty sure the Motion Controller doesn’t do M mode. Page 10 of your manual.

Scroll through the various settings menus to find how to turn manual mode on. Or you could read the instructions.
Manual mode has some hazards which DJI doesn't want accidentally turned on. There are several stories in the forum of people turning their motors off in while in manual. Make sure you know what you're doing before you enable it from the menu.

Duane Degn Posted at 4-10 21:03
Scroll through the various settings menus to find how to turn manual mode on. Or you could read the instructions.
Manual mode has some hazards which DJI doesn't want accidentally turned on. There are several stories in the forum of people turning their motors off in while in manual. Make sure you know what you're doing before you enable it from the menu.

There’s no manual mode for the Motion Controller. Only Normal and Sport.

"There’s no manual mode for the Motion Controller."
The Motion Controller wasn't mentioned.
After rereading the original post, I think you're correct in your assumption.
To me, it wouldn't make much sense on having a manual mode with the Motion Controller since you don't have the same level of control as with the normal controller. At least I assume you don't have as much control. I haven't used the Motion Controller.

Hi, WinstonSmith. Thanks for reaching out and posting your inquiry. Currently, the DJI Motion Controller does not support Manual Mode.

DJI Gamora Posted at 4-11 23:04
Hi, WinstonSmith. Thanks for reaching out and posting your inquiry. Currently, the DJI Motion Controller does not support Manual Mode.

Would DJI allow an update that permits full flight speed for the motion controller? I understand the maneuverability limitations with the motion controller but I’d love to use it to fly 80+ mph.

If you wanted to flip, you'd have to do a cartwheel while holding the controller.