Hi, Smartie. For 39d, the word "Rover" needs to be removed, and the wordplay is "driving company" (giving AA) and "200" (giving H - this is one of those medieval Roman numerals that often pop up in Listener and other advanced crosswords).
For 46a, the word "using" needs to be removed. Think of a four-letter word that you might use for "utter" in a sentence like "it was utter delight", and drop the last letter (implied by "mostly"). The "dog letter" was part of the theme of Listener 4381 back in January, otherwise I would not have known which letter this is. Despite the reference to dogs in the clue, the answer is a sound associated with a quite different pet.