Interesting... the difference between English language as well as English law and the American version of each.
Here, in the U.S., a subpoena must be served by an officer of the court (usually a Deputy Sheriff) for which the deputy reports back to the court the subpoena was served. After that, the court can order you arressted to compel appearance. A letter would not be considered a subpoena.
It'd be illuminating what the letter says as far as the court's expectations, regardless of your desires to not appear...