Using Anglo Saxon Accounting

When create PI from PO by clicking Invoice button on PO the amount due is showing the tax amount and not the Untaxed (Net) amount + Taxed Amount.

When I bring up the bill Accounting>Purchases>Bill Payments and click Register Payment the dialog box pops up with the tax amount in the Payment Amount, if I overwrite with total invoice amount the payment difference field shows a minus amount to the value of the net amount. If I click keep open radio button  and validate get warning message: Entries are not of the same account!.

If I try Mark Invoice as fully paid radio button a Post Difference In drop down box appears giving nominal account options.

What do I have to do to get the correct Amount Due value?

The journal postings to the Creditor, Interim and Purchase Tax accounts are correct for PI creation - has the contra posting to the Interim account been taken as payment?