The contestee shall, within ten days after the complaint is served on him, file in the clerk’s office an answer, in which he shall admit or deny the allegations on which the contestant relies, or state that he has no knowledge or information concerning an allegation which shall be deemed denial, and state any other defenses, in law or fact, on which he relies. If no answer is filed within the time prescribed, the contestee shall not be heard to assert any claim or objection which is required by this section to be stated in the answer. No enlargement or amendment of the answer, except as to form, shall be permitted save by leave of court as provided in Rule 1:8 of the Rules of the Supreme Court of Virginia.