1. Ring Pension Service and ask to speak to a manager. Tell them you wish to make a formal complaint about the standard of service and ask to be sent details of the complaints procedure. When you get it, complain in writing and follow it through. Hopefully, the initial phone call will result in someone taking ownership of the problem and sorting it out. Keep a copy of all correspondence and send it recorded delivery.
2. Alternatively, or as well, go back to your MP and ask him/her to chase them up again. If use of the complaints procedure does not work you will need to go to the MP again anyway, as he/she can then refer the complaint to the Parliamentary Ombudsman (but not until the internal complaints procedure has been exhausted).