No I was wrong,

I have just noticed that I have the following stock:

Small = 4

Medium = 0

Large = 40

I have done what I mentioned above and set:
Line #1146 : case ($button_check->fields['products_quantity'] <= 30 and SHOW_PRODUCTS_SOLD_OUT_IMAGE == '1')

But this has still just allowed someone to check out when the Small was set to 4!!

Please someone point me in the right direction.. if the attributes disappear when they hit 0, there must be somewhere I can change this value to 30....