IVA and HRMC child benefit overpayment any advice please?

Hi
I am starting the process of an IVA. I have an overpayment of child benefit which I am paying back through my tax code. I am a PAYE worker but do a tax return for salary sacrifice car scheme and also child benefit. However as I need to submit a tax return each year I haven't completed the tax return for 16/17 and I will also owe a small amount from Apr to Aug 2017 which will be in the following years tax return ( 2018) Step change have told me I need to contact HRMC direct to see how I should manage my tax returns . Any advice on who to contact? Has anyone had this issue and how have you dealt with it? SC has said I can say I am looking at an IVA but as I am new to this I am a little frightened of having that conversation. Any help is appreciated. This is my first post on the forum but I have received a lot of valuable information from the forum and I hope someone can help. Cheers LJmini