Thanks a million Steve!
I hadn't found that report to Eugene previously but it does actually correspond fairly well with the version in Macdonald's Souvenirs, apart from being longer and rather more dramatic. Conversely, looking at the report of the action of the previous evening, I note that he plays down the disorderly retreat of his troops on that occasion though in the Souvenirs he emphasises what a near disaster it was.
Presumably the reports at the time were intended for the eyes of the Emperor and possibly for publication while his later version was for the information of his son only. This shows that the usual assumption that the report written at the time will be more reliable cannot be taken as an absolute rule.