Promotion Help

I want to create what seems to me to be quite a simple request but I’m unable to get my head around how to achieve it after several attempts.

We want to offer certain products (specifically Heathers, which has it’s own category) at a discount price if someone purchase 10 or more. Obviously I can set this up on a product by product basis with the ‘Quantity Discount’ option but we want them to be able to pick and mix from all varieties in the category rather than 10 of a particular variety.

My predicessor set up something similar for 25 heathers but when I looked at that there was a problem which was that you could add 25 heathers and it would knock off £1.00 of each heather but if 24 of the heathers we’re removed but their was 24 other items in the cart the single heather left would still have the £1.00 discount applier, so it seems the cart can count products as a whole but can’t count products from a specific category.

Could anyone offer any advice?