Thank you for your suggestions.
I was unable to find any Parliamentary papers that covered
the Napoleonic period. I also spent some time going through the Wellington's Dispatches and Supplementary Despatches available via
Google without finding a pertinent item. I did find correspondence with Torrren in which Wellington complained of officers being sent to him
who had no experience and his preference of promoting officers
to vacancies from those already with him in the field.
Even if I can’t find a citable reference, your experienced explanation
certainly explains Cockburn‘s statements about his leaving the Sicilian staff
and guides my understanding of the movements of Cockburn and another general in late summer and early fall of 1810. Thanks again.