How to add sort by "quantity" for product list?

[dunngra]

Hi there,

 

Would somebody be able to help me with how to add sort by "quantity" onto the "default order by" options for product preferences?

 

I simply want to be able to default any product list to display the items with the highest stock levels first in descending order.

 

Many thanks

 

P.S I'm using 1.4.9.0