In cell drop down based on a filtered table

[mummbles74]

I am creating a spread sheet to track our PO's and there remaining values and create our invoices each month. The part that I am having difficulty with at the moment is creating a cell validation list based on filtered data. As an example, On sheet 1 there is a list of PO's in Column A in column B there is a list of Companies/clients. On sheet 2 (invoice list) I want to be able to pick the client in column A and for this to filter an in cell drop down list of all the PO numbers against that client in Column B.

Hopefully that is a good enough explanation of what is required. I do not ind if the solution is VBA or not.