MP and CSA side against me to take additional £7k

In 2015 csa wrote to me stating i owed £7306.36p.i had ended my monthly maintenance payments.i wrote via mp philip hammond who took no interest in my plight and sided with csa.after 1yr of letters i had to pay the 7k. And appointed external audit co.to check all my csa accounts.after short period they found csa error 7k and since then which has been 6 – 8 months they have tried to get csa and new mcs to admit their error.the csa refuse to co-operate.Meanwhile at age 65 am still awaiting csa reply.