How to rotate OCAD map to match georeferenced background maps (OS open data in my case), removing existing MN rotation.

1) Open georeferenced background maps
2) Select
         All
           Move objects to as correct position as posssible
3) Calculate rotation by comparing objects to background map
    If zero > exit
            
4) Background map
           Manage
                   Remove all
5) Map
      Transform
              Rotate
                (enter angle)
6) Map
       Set Scale and  Coordinate system 
               Real World Coordinate System
                 set angle to zero
7)  Goto 1

Now put back in MN...

    Map
         Transform
             Rotate