I am getting there now.

I can now change values in the cart from my extension.

Of all the hooks, I think before_save_cart is the best to use as in both before_add_to_cart and before_update_cart change the values that I change in my extension, therefore my changes are overwritten.

At the moment I am working with the quantity value. I change this with the before_save_cart hook, however, I must update the cart to see my changes. A simple refresh of the page does not work, and I cant call update_cart from my extension as I get an infinite loop (as it is called again from the same before_save_cart hook).

I am going to think about this, but any insight would be greatly appreciated.

Thanks,

Leon