Hi Peter,
I ordered the documents you referenced and saw them at Kew on Friday .
HO 99/18 is a hard-back book titled “Isle of Man Entry Book” where letters written from Whitehall to the Lt Governor of the Isle of Man, starting 20th May 1830, had been copied by hand.
HO 98/77: This box contains the actual letters from the Lt Governor of the IOM to Whitehall, the answers to which are copied in the 99/18 book. Included is one dated 25th October 1833 re "expense of conveying one Female convict" (not named). The letter with the latest date in the box is one re the supplying of water to Douglas - this had a 15th November reply in the HO 99/18 book, immediately before a reply about the cost of prosecuting Sarah & Thomas Linchey. I hoped therefore that the first letter in HO 98/78 would be the letter to match that reply. But no.
HO 98/78: This box mainly contained a large collection of letters from various people, concerning the plundering of the wreck of the Fairfield, from March onwards in 1835, but there were also a few other letters on random topics. There was also a list of prisoners dated 13th April 1835 – all male, no Linchey.
I may add that there was also in one of these boxes a letter from Jersey concerning Castle Elizabeth in Jersey, on which someone had pencilled “Isle of Man?” which gives you a bit of a sinking feeling.
The Isle of Man boxes in HO 98 seem to be 63-82, though I must say the catalogue doesn't seem to be flawless.
In a new message I'll copy some letters from the HO99/18 Entry Book, around the dates and page numbers mentioned (though I think the pages refered to would have been in the Manx entry book).
Not the answer you were hoping for. It was most frustrating that the set of letters in 98/77 seemed to stop right before the one we are looking for!