Hello Roy - very pleased to meet you. I also think First Mate was the highest rank Thomas rose to, having seen some of the documents.  I am about to rush off to work but would love to share what I have, and am fascinated about the info from the Merseyside Maritime museum.  Verbal history in our family said he was a captain, (of a slave ship) but it does appear that was exaggerated.  I will DM you when I get back from work and send my email address.  Kind regards, Alison.