Cascading validation

I have a stock purchase order sheet that I want to make easy for the people in the office to use.
There is a sheet with all the cost tables on ('Costs'), a sheet with all the names and addresses etc of the suppliers ('Supplier List') and then the order form itself ('1').
On the order sheet I have a drop down box that selects the supplier and adds their details to the cells below it.
There is then a column that I want to have drop downs in showing only the codes that correspond with the selected supplier. When a code is picked, I then want the description and cost cells to be automatically filled in. This leaves the Qty column to be completed by the user. There are tables and names defined on the workbook for all the items.
I have looked around the net and tried a few of the examples but nothing seems to work.

Can anyone have a look and point me in the right direction?
Kieran