Hello Jim Lair,
I came across your message whilst browsing Robert Dinwiddie who I have always been led to believe is an ancestor, probably great, great grandfather. I can only help you with vague information (i.e. on my side) his daughter married a David King who had Gibraltar connections, he (David) was father of my Grandmother Marian or Mary Anne King (Minnie)born Feb 8th 1859. She married Dr Henry Mainwaring Holt. They had my father Harry.
I have a black and white portrait (rather small) and a miniature of Robert Dinwiddie. The portrait is not the same as the one I have come across often on the net, but it is without doubt him and is signed by him unerneath (I don't think it can be an original). Tantalisingly, my father had stuck a small envelope to the back of it saying 'notes on Robert Dinwiddie esq.' sadly the envelope is empty!
Please bear in mind that much of this is family legend and hasn't been verified.
I DO hope you get this you've been waiting for a response for a long time!
Margaret Miles nee Mainwaring Holt