Think I have sorted this, found on the internet that if you check the actuator without it fitted it will turn both ways but stop on the mechanicle stops on the unit. These stops take it outside the control of the computer. When it is fitted into the HVAC unit the blend door travel stops it first. Just got to see if I can fit it in such a confined space, don't really want to have the dash taken out again.
1999 Chevy Blazer 4.3 ltr RHD converted to LPG