Among the case papers of William Ashurst (a judge of the King’s Bench and a contemporary of Mansfield) are a set of papers relating to Somerset’s case. The documents include a copy of the return to the writ of habeas corpus. (This was the response written by John Knowles, the captain of the ship on which Somerset had been imprisoned justifying his actions).
Most of the set of papers are handwritten notes of Lord Mansfield’s opinion as it was being delivered. This version of Mansfield’s opinion omits some wording included in other texts, including the printed report but this is probably attributable to the fact that the author was writing in long-hand.