Mc doess indeed insert the heating gcode automatically. It does this just before it runs your customised start gcode. The only exception to this is if your custom start gcode has heating code in it... the mc figures u are taking care of it and it no longer has to do it automatically. On some printers heating before homing can cause issues so I did try the heating code in the custom start code but it made no difference to my other issues and given it is a dual extruder... easier to let mc take care of heating and picking extruders.
Thanks for the feedback