Pontious N
It sayeth in the Bible, JSP101 Ch 2 Para 11;
"Personal Letters. The choice of salutation in a personal letter depends on the relative ranks or grades of the writer and addressee and how well they know each other. If you are writing to an equal or junior that you know well, use the recipient’s forename or ‘nickname’. If you are writing to an equal or junior that you do not know well, use the format ‘Dear Squadron Leader Bloggs’. This format is also recommended when writing to a superior, eg ‘Dear Colonel Franks’ or ‘Dear Mrs Smith’ (and use the generic rank for military officers; eg refer to the rank of air vice-marshal or rear admiral as ‘Air Marshal’ or ‘Admiral’ respectively). The valediction is a matter of personal choice but ‘Yours sincerely’ is safe and recommended".