Minimum quantity & multiples

Does anyone know of a module that allows the admin to insist that a product is purchased in multiples of a certain amount?

ie, a multiple of 5 requires a customer to purchase 5, 10, 15, 20 etc products.

I am aware that OC already has the facility to require a minimum quantity to purchase, (ie 5), but after that, a customer can purchase 6, 7, 8 etc.

Many thanks

in 1.4.x, this would have helped:
http://forum.opencart.com/viewtopic.php ... t=0#p86666

But in 1.5.x the system is vastly different so it would take some research

Thanks. I intend to use 1.5.x though :/