Product navigation only works when sorting by "Product Name"

[babynose]

Hi,
I have been trying to work out why the product navigation was missing on my VirtueMart website.
After a lot of digging it appears the main reason is that the product navigation is dependent on the "Default Product Sort order" being "Product Name".
In "Configuration"->"Product Order Settings" if the  "Default Product sort order" is changed to "Product SKU" the product neighbors disappear in product details.

I am using Joomla 2.5.9, VirtueMart 2.0.18a, PHP 5.3.19

Regards,
Mr BabyNose