Blanket Orders- Purchasing

Hi,
Can someone kindly explain the concept behind a Blanket purchase order in Nav?

My business need is to create a BO for say 10,000kg of a item, which we can then give to the supplier to hold the price and to have stock of.
It has no impact from a MRP point of view as it it not seen as a supply.

When the Purchasing Officer raises a Purchase Order, somehow the qty order/received/invoiced needs to linked back to the Blanket Order.
Typically they will raise these orders for smaller qty's over a period of time.
Each Purchase order/line will have different due dates.
This then allows us to manage the supplier so see how much we have ordered against the original BO. i.e. if 2 PO's are raised of 2,000kg, then the BO will show the qty raised/received/invoiced.

In Nav 5 sp1, when I create a BO and then click function make order, a PO is created for the whole qty.
I only want to raise a order for say 1000 this week and then again when I have a demand for it on a new Purchase order.

How do I do that or is there a key concept that I'm missing in Nav?

Thanks
Fred