Cracked it: See Scottish Gypsies under the Stewarts by David MacRitchie. Apparently William Marshall was b1670, and dNov 1792, which made him over 120 years old. I am very surprised that thre is not more information given on such a character, but Googling William Marshall Kirkcudbright gave this information, and a lot more, too much to reproduce here. So fascinating I might even buy the book, and I lived over 40 years in Yorkshire!!