IVA calculated wrong

Hi it's my first post. My IVA is finished Nov 18. I pay £180 a month now after getting behind when on maternity leave. When I first started the IVA on the statement it said I owed tax credit £3600! I called and said I don't even receive this is tax credits in a year so how can I owe this? I had to get proof from them in writing and the IVA said it would be removed. 3 years on and I have got my first statement and this is still showing again I had to resend the letter to them. I called last month when had to set up higher payment agreement and I was told the £3600 was still on as a creditor! They said they have the proof on file and an error as still on and will get it removed. I asked now if my payment would be less as this was a quarter of my IVA balance and I was told I still need to pay the same payments even though IVA totally calculated wrong and they will just share the extra money I pay between the other creditors. Can anyone help and is this correct? I would of paid £7150 of £12000 debt.i am so frustrated as want to clear the IVA to buy my home as mortgage be less then rent but this is whole new problem!! I need a good mortgage advisor if any recommendations or tips. Thankyou in advance