Cheers Matthew for trying
Found another great way to do it....
there is a workaround....
Make a copy of your template and remove the module position that you don't want to show...
edit the template.xml file so it will install as a new template with a new name from the original...
then assign that template to the menu items that you don't want that module position to show on!
you can then change the assignment of the module in question to all and it will not show on those menu items you don't want them to because those positions don't exist in the template!