If any difference arises as to the amount to be paid under this Policy (liability being otherwise admitted), such difference shall be referred to the decision of an arbitrator to be appointed in writing by the parties in difference or, if they cannot agree upon a single arbitrator, to the decision of two arbitrators, one to be appointed in writing by each of the parties within one calendar month after having been required in writing so to do by either of the parties, or, in case the arbitrators do not agree, of an umpire to be appointed in writing by the arbitrators before the latter enter upon the reference. The umpire shall sit with the arbitrators and preside at their meetings. The making of an award shall be a condition precede to any right of on against the Insurers.