Andy,
I was one of the pilots on 825. HRH did not fly with us as I recall and unfortunately I can't identify whether 91 was one of ours or not. It certainly was originally a 706NAS aircraft but again I vaguely recall one or two of our training aircraft being sent to Portsmouth to embark on one of the carriers as extra aircraft.
If you had the a/c registration number I may be able to refer to my logbook.
It does look like HRH but in the wrong aircraft unless 820 NAS had a utility from our number as explained above.