Anyone with experience paying back tax credits?

[AlaskaThunderfuckHiiiiiiiii]

Long story short we owe a fair bit back in tax credits and so are paying this back through advantis at £40 per month, we received another letter stating we owe another small amount from another year and so contacted them to add this on to the current repayment as we cannot afford anymore than the £40 we are currently paying, they are saying they cannot combine the two even though it’s to the same people HMRC, it’s not going to make any difference splitting it as we cannot afford anymore where do we stand? I feel I have made an effort to address it by asking for it to be added on and to continue paying as we have been?

I've got the same thing so just wrote to them and told them to add it to the other payment plan and kept copies.... I get the annual statements but nothing else at the moment. I never talk on the phone, always by email or letter