Thanks for your reply Renatas, it makes things clearer.
The G/L account for the loan, historically, has been set up as open-item-managed. Perhaps it should not have been because, as you say, the Tresury module is automating the postings. I may suggest to them that they change this setting
This issue has now been re-assigned to my client's Global help-desk, wher they have some Treasury consultants, so hopefully they will be able to resolve the issue.
Thanks for your assistance.
Marc