Thanks for all the answers.
I must have done something wrong when testing G91 G28 C0, because I can make that work, but it uses Mach3 homing setting and not the more precise smoothstepper homing - backing off the sensor at another speed.
When playing with the above, I found out that it homed at another speed and did not back of the sensor, and then it hit me, could it be a smoothstepper homing bug? Yes it is, if I disable the smoothstepper homing settings it works as expected.
I will contact warp9 and tell them about the bug.
Once again, thanks for all your solutions and help, this is a fantastic place.
Regards Morten.