Auditing changes of VAT codes between PO and AP PO Invoice

Summary: Report of differences of VAT codes between Invoice lines and matched PO.

Content (required): I am trying to create a report of any invoices that have a different VAT code to the PO that they are matched to. What I am finding is that a lot of the PO Lines do not carry any kind of VAT code, so the differences that are there may not represent the full scope of the differences.

What I am looking for is the missing VAT codes, can anyone give me any pointers on Procurement tables where I am likely to see what I am not seeing on PO Line Locations?