Thanks very much....This is now developing into world war 3 as on the message board I use for my local football team Brighton, we have university professors and teachers arguing with each other about the correct usage.
The consensus is that although the name was not used, as the letter is directed at the Deputy himself and not the Office of the Deputy, then it is "sincerely"
It hasn't come to fisticuffs but I can see it getting there!

))