So I have just tried to implement it, I was hoping it would be able to stack the different coupons that you suggested.
However, the first stumbling block was the coupons all need unique names! So I have had to create three separate coupons:
2 for £15
4 for £30
6 for £45
I added 2 items to the cart, and it used the first coupon. I added another 2 and it changed to "4 for £30" (which is not really the name of the promotion, but i'll work with it if I need to) which is expected. I didnt have enough stock to add another 2, so I ended up adding 5 of another item.
As you can see below, having 5 of the two different items in the basket doesnt work. The items are both from the same category, and the coupons are all for that specific category.
http://screencast.com/t/jFhRYAfZIW
I have seen AWOCoupon will plugin and support the functionality we need, but i'm not sure its fair to advise my customer to pay for another plugin when this is a really basic fundamental requirement for an online shop?