Alas, I misrepresented the issue. The reported error was a red herring as the data was not in fact the same as the previous iterations.
It was special & unique & could be detected with
'Error Area = Session.findById("wnd[0]/sbar/").MessageId
'Error Number = Session.findById("wnd[0]/sbar/").MessageNumber
'Error Type = Session.findById("wnd[0]/sbar/").MessageType
to find the error.
I shouldn't have assumed it was a timing issue.
Thanks nonetheless 