Hi Dean,
Sorry for the delay - I hadn't found enough time to investigate this...
The NETVAL and NTCUR fields are populated in the Function Module /SAPSLL/PRN_VIEW_GET, in form VALUE_TOTAL_GET (just in case you're interested).
The values are taken from the original value & currency for the Net Value Duty Type in the header (VALTY = '001'). If no such value already exists in the header, the sum of the items is used instead.
That Function Module runs during the Incompletion Check and the Message sending, so if you haven't yet checked or sent any messages, the fields will be empty.
Hope that helps.
Regards,
Dave