Andy - you don't say the main mixing bowl of what?
The general rule is that things unscrew anti-clockwise - my Kitchen Aid mixer - the bowl on that unscrews anti-clockwise. The mixing arm rotates the other way as if it were trying to screw up the bowl.
When I have trouble with stubborn bottle and jar tops I usually ask a child to do t for me - or the wife - it always works.
Good luck...
Peccavi
January 2013