Next / previous post link within ‘Parent’ category

Dear forum members

I hope you can help me out with this one. I’ve searched high and low in the forum but couldn’t get any answer to this one issue I have.

I would really like to know how do I use “next_post_link” (or any other method) to display the next post within the parent category instead of the next post within the category (<?php next_post_link(‘%link’, ‘Next post in category’, TRUE); ?>).

To help establish context…
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I have sorta hacked WP into acting like a CMS where the parent categories, eg- Articles, Blogs, Downloads are parent categories. I post in each of these categories upon various topics which are subcategories.

Eg:
Parent Category: Blog
Sub categories: Animation, drawing, painting

When a user clicks upon a post in the ‘Single’ page that is posted under ‘Animation’, I would like that previous/ next post take the user to the next post within ‘Blog’ and not ‘Animation’.
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I am really hopeful to see a solution. With guys like Moshu and Kaf, and a load of helpful others, WP sure manages to attract a lot of adopters where community support acts as the major differentiator/ reason for adoption.

Thanks
C