the PRO is at http://catalogue.pro.gov.uk/
plugging Qualtrough into search engine returns 5 entries in their on-line catalogue
3 army (post 1915); 1 Home office (ecclesiatical appointment) + 1 Board of trade (looks like appointment of harbourmaster)
But you have probably been here
(I referenced this at www.manxnotebook.com/history/military/ms_1850.htm

you will however have to pay a personal visit (or pay one of the many professional serchers who work there) to see documents