How do I default the tax classification code from the AP invoice line?

I'm implementing Cloud Payables and have to do the related tax setup for UK VAT.

The requirements should be fairly simple; allow the user to enter a tax classification code on the AP Invoice line and use this to calculate the tax for that line.

I have completed the setup steps as I understand them (listed below) and when I manually enter a tax code in the :

Manage Tax RegimesManage TaxesManage Tax StatusesManage Tax JurisdictionsManage Tax Recovery RatesManage Tax RatesManage Configuration Owner Tax OptionsManage Tax Determining Factor SetManage Tax Condition SetDetermine Tax Applicability RulesManage Tax Rate Determination Rules

However when I create an invoice line, assign a tax classification code and then select 'Calculate Tax' no tax line is created.