While I have been unable to confirm this marriage I have no doubt that it must have occurred as her brother Captain James Cook refers to her in this way in his Last Will & Testament dated 14th June 1776 prior to his departure on his third and last voyage 25th June 1776
Proved at London the 24th day of January 1780.
"I give to each of my sisters Christiana Cocker and Margaret Fleck the sum or legacy of Ten Pounds"
