Only the destination document needs the INCLUDETEXT field. You can approach the problem in either of two ways:

1. Have a source document listing both Tesco and Sainsburys, with both Tesco and Sainsburys bookmarked appropriately. Then, in each destination document, have an INCLUDETEXT field coded along the lines of:
{INCLUDETEXT “C:\\Users\\Pascual\\Desktop\\customers.docx” Tesco}, or
{INCLUDETEXT “C:\\Users\\Pascual\\Desktop\\customers.docx” Sainsburys}
as applicable; or

2. Have a separate source document for each company. Then, in each destination document, have an INCLUDETEXT field coded along the lines of:
{INCLUDETEXT "C:\\Users\\Pascual\\Desktop\\all the customers\\Tesco.docx"}, or
{INCLUDETEXT "C:\\Users\\Pascual\\Desktop\\all the customers\\Sainsburys.docx"}

You may also find the relative paths discussion in https://www.msofficeforums.com/word/...nal-files.html useful.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]