5 for 4 offers problem when buying 10?

[rayr]

I have set up a 5 for 4 offer using the discounts & surcharges dialog with quantity =5 as the trigger, and 100% off the cheapest as the reward.
If the customer purchases 5, no problem basket charges for 4 but if the customer purchases 10 they are charged for 9 !!
I have the ' Only one reward per order, ticked is this stopping the logical buy 10 pay for 8 happening?

Can anyone help with this??
Thanks

[wjcampbe]

In a word (but the forum needs ten characters to post) - Yes