Running 1.3.9h

I am selling only download products. I have the stock initialised to a notional 1000 and have:

Config ==> Stock:
Check stock level true
Subtract stock true
Allow Checkout true

For an individual product the basis is pricing by Attribute and price depends on size (S/M/L/XL related to the w x h in pixels). Customers are invited to purchase one of the size that will meet their needs. There is no point in purchasing more than 1 product.

In the product I have:
Products Price (Net): 0.00
Product is Virtual: no
Products Quantity Box Shows: Yes, Show Quantity Box
Product Qty Minimum: 1
Product Qty Maximum: 1
Product Qty Units: 1
Product Qty Min/Unit Mix: No

My problem is that it is still possible to add more than 1 of a product and when that happens there is no quantity box (so the quantity can be adjusted down) in the "Shopping Cart Contents" screen so the only option is to delete the item and start again.

The Max: 1 shows against each product added.

Q: How can I force the max of a product to be 1?

Thanks,
Ric