Primary Category shows as SEO URL & Breadcrumb

Hi there,
What i want is for all my products to have a primary category, which will show as the url as well as the breadcrumb trail.
This means that whereever the user has browsed to the product from, the url and the breadcrumb will show as the primary category.
The products can still be put into other categorys, but when clicked it will link to the primary.
Probably a 5 minute job for anyone who knows what they are doing.

I believe the version of opencart that i use is 1.3.4

PM me or post here

Cheers