Still no payment

As a service provider, I submitted a claim to Commed at the end of December 2015 for a patient treated in hospital. After resubmitting every few days for weeks with no response, I phoned and was told they only received the claim in the first week of February. I have no idea how this is possible, that an email sent repeatedly to the identical address is only received after 4-5 weeks. Nevertheless, I was told the claim had been processed for payment and I would be paid within 6 weeks. Needless to say, 6 weeks have come and gone so I called again today. I was again told the claim had been processed for payment, but no payment date had been sent. The claim is now over 3 months old and I am just waiting to be told it is too old to be paid. What else do I need to do just to get paid what is legitimately owed to me?