I did some digging around the company database looking for the correct way to get the next Journal Entry number and found a function called glGetNextJournalEntry.
A little work and I was able to supply the following to the developer for him to wrap into a stored procedure to get the Journal Entry number and make sure there were no issues in GP;
Created by Ian Grieve of azurecurve|Ramblings of a Dynamics GP Consultant (http://www.azurecurve.co.uk) This code is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0 Int). */ DECLARE @l_tINCheckWORKFiles tinyint = 1
DECLARE @I_iSQLSessionID int = USER_SID()
DECLARE @O_tOUTOK tinyint
DECLARE @IO_iOUTJournalEntry int = 1
DECLARE @O_iErrorState int
SELECT @IO_iOUTJournalEntry AS 'NJRNLENT', @O_tOUTOK AS 'OUTOK', @O_iErrorState AS 'ERROR'