The forensic scientist tells Morse that she put the threatening letter on a specific but unnamed machine to reveal the writing on the previous sheet of the writing pad. The machine used is an Electrostatic Detection Apparatus or ESDA machine, which uses electrostatic attraction of toner powder to highlight indentations in the paper.
The book that Michael Steppings reads to his comatose daughter in the hospital is Black Beauty by Anna Sewell. The pony referred to is Merrylegs.
Ian McNeice would later turn up in Inspector Lewis (2006) the spin off series as a clergyman. As would Jason Durr as a DI
Brian Cox and Jason Durr appeared, separately, in episodes of Sharpe - Cox as Major Hogan in Sharpe's Rifles (1993), and Sharpe's Eagle (1993) and Durr as Colonel. The Lord Keily in Sharpe's Battle (1995). Sean Bean, who played Richard Sharpe, himself appeared in the Inspector Morse episode Absolute Conviction (1992) as Alex Bailey