By OFGEM rules, he or an appointed deputy, MUST respond to any form of communication that is a complaint. Still, since he received the original communication, he must ensure that it is closed off to your satisfaction. It is also good corporate practice. Remember how quickly you got a response from Scottish Power - that was an example of how it is done properly.
Should you get no response from Derivaz, then we can escalate it to OFGEM who also keep league tables and heavily fine those who don't respond quickly and with a result that keeps the consumer happy. IIRC, EDF are top of that table.
Colin