Your message seems a little convoluted - you have sent a letter via Royal Mail to person A; this letter is received by person B who neglects to pass it on to person A; it then ends up in the possession of person C and is ultimately removed from person C by person D who then says "letter ? What letter ?" to person A; this is my understanding of it.
Did person B intentionally prevent person A receiving your letter ? If so, can you prove this? You may well find that the interest from the authorities in any 'postal crime' is not worth your trouble in contacting them.
You could write a strongly worded letter to person B and state that you have sent a copy to person A. You may be best placed to either hand this letter to person A yourself or send it to a postal address elsewhere.