Change VAT code in Purchase Order based upon Date

Hi Colleagues,

I have a problem with the change of VAT here in the UK that took place in December

My client utilises third party sales order processing, however, they do not instigate the sales order process until the supplier submits the invoice

These invoices can be dated pre the tax code change in the UK. Now on the sales order side this is fine because the MWST condition is found based upon the services rendered date

Example: 29.11.08 services rendered finds A1 at 17.5%

02.12.08 service rendered finds the new tax code at 15.0%

The third party order contains an item category of TAS which then allows the creation of purchase requisition and automatic conversion to purchase order. The purchase order derives the tax code from the information record, which is now set to the new tax code

Therefore on a pre-December invoice the sales side from a tax perspective is correct, but the purchase order side is incorrect

Is there a way of automatically deriving the tax code in the purchasing document based upon delivery date

Thanks

Steve