They were on the Stirling Castle as early as 31 March 1843
I think you are probably being misled here by the rather confusing nature of the document. HO 8 Piece 81 is a volume comprising of the returns from the various hulks. I assume that returns were sent from each hulk and then copied by clerks in the Home Office into the volumes which are now held in the National Archives. This particular Piece (the record of the quarter ending 30 September 1844) begins with the records from “Stirling Castle” at Devonport (images 3, 4 on Findmypast in the browser). The section devoted to this hulk is followed by the records for the same quarter from “Leviathan” at Portsmouth (images 15, 16). John Sayers, prisoner 6625, and William Sayers, prisoner 6626, appear on image 22.
The press reports are indeed fascinating. The pursuit of the donkey cart by the policeman in disguise and the discovery of the soaking wet clothing hidden under a smock frock in the band box under the bed, oozing water, are highlights. I have two questions: how did John Sayers see where he was going as he rode his cart (maybe he ate carrots, or did they simply choose a clear night with a pretty full moon?) and why on earth did William hide his wet trousers and not simply hang them on a washing line, as he surely would have if they’d become wet whilst fishing? Maybe the suggestions of a drink problem referred to by his employer are close to the mark.
Dave