Hi Alastair,
I believe you are trying to post customs items, where the amount charged in an invoice or the items in an invoice are custom items. For this you can choose
1. In the supplier invoicing work center > select New Customs Invoice, in this you can enter items which are 100 % vat, you need to enter the expected tax code and VAT amount, this amount is posted as VAT.
2. In case you have invoice, where you have mixed items like which are 100 % VAT and other invoice items, you can post it directly in a supplier invoice, you need to change the invoice item type for those VAT Entries. The invoice item type can be checked via personalize, item type.
Then it is possible to choose 'Import VAT debit item' or 'Import VAT Credit item' according to you posting requirement.
Thanks
Lokesh Sharma