repayment on wftc overpayment increased without explanation or notification

I had an overpayment on my HMRC WFTC which is being repaid at £35 pm from jan 2018 and is due to end next year , this month HMRC have taken £38.14 without notifying me of any change nor providing any explanation. As I understood it the repayments were fixed when they were agreed , now all of a sudden the payment has changed can someone please explain this ?

Hi ts962,

You will have to contact the repayment helpline to discuss, their number is  0345 302 1429  
I am assuming that it is WTC not WFTC which was replaced in 2003.

Thank you.