Memorised Invoices problem

Sage 50 Accounts Plus 2008 v14

The set up:

Products have different VAT rates: some standard, most zero-rated

The Customer, therefore, has the "Use Default Tax Code for Sales" box unticked so that the invoice picks up the tax code from the product record.

The problem:

Using memorised invoices in Invoicing, (as most orders contain certain core items) as soon as the A/C code is entered, the VAT on all items changes to the default code for that customer. But, as stated above, the "Use Default Tax Code for Sales" box is unticked!?

Does anyone have a workaround to this or is this just another Sageism that we have to live with?