Thanks for all your replies - its potentially a minefield with so many different options/switches to consider. For those that may be interested this scenario seems to do the trick:
Scenario 1 – Combine Items onto a Single Purchase Order
3 sales orders on system all have ITEM “JW2010”
SOR0001 - JW2010 – QTY 100 – DUE 06/01/2010
SOR0002 - JW2010 – QTY 100 – DUE 06/01/2010
SOR0003 - JW2010 – QTY 100 – DUE 07/01/2010
Item Card Settings example
In stock = 50
Reordering Policy = Lot-for-Lot
Reorder Cycle= 30D (this is number of days)
Safety Lead Time = 10D (this is optional)
Safety Stock Qty = 30
Go to Requisition Worksheets
Click Functions – Calculate Plan
Item Tab – No. = JW20101 Options Tab – Start date 06/01/2010 – End Date 07/01/2010 – Click OK.
This returns 1 line suggesting an order for QTY 280 – which is (SO = 300)+(safety Stock =30)-Instock = 50.