Changing Tax Amount on Invoice via Web Service

I’m currently saving Purchase Invoices into our NAV 2009 (not R2) instance using the Purchase Invoice Page web service.

This works correctly, however, I want to be able to update the Tax Amount (GST Amount in our case, which is VAT_Amount in the table) that is present when you press F7 on a invoice line item.

I’ve tried looking at the Purchase Invoice Statistics page, but that has no Update method, and the Lines page for the VAT Specification Subform doesn’t return any data and I cannot filter on the line item or invoice id.

Is there any way to update this field using a web service? Note, we do not have a developer license so I have to use Page web services only.