Approached from a different angle and think I've cracked it :)
DEFINE VARIABLE cRecType AS CHARACTER NO-UNDO.
DEFINE VARIABLE cIDSender1 AS CHARACTER NO-UNDO.
DEFINE VARIABLE cInvcNum AS CHARACTER NO-UNDO.
output to "E:\Epicor\TMat Custom Info\Data Export Logs\Invoice.txt" append.
For First ttInvcHead no-lock:
If Available ttInvcHead Then DO:
cRecType = "BGM".
cInvcNum = String(ttInvcHead.InvoiceNum).
PUT UNFORMATTED
cRecType AT 1
cIDSender1 AT 4
cInvcNum AT 8.