Maintenance payments unreasonable

[namechnge404]

My partner received a letter telling him his payments for his daughter is increasing to cover previous arrears. The problem is it leaves him with about £500 to pay towards his son and to live on for the rest of the month. I have said for him to appeal. He is more than happy to pay its just the amount he has an issue with because he just cant manage.

Anybody have any advice?

Should say we dont live together so its not like he is paying for me or my children

Why did he fall behind in payments?

Can he manage the normal 20% usually and it's just the arrears he's struggling with?

They are taking 60%. The actual payments were increased by £50 a month even though his wages haven't increased. It is the arrears he is struggling with. They just told him a few months ago there were arrears. They say he had been underpaying for 14 years.

He pays for his son privately.
The letter says he should be left with £600 which would be difficult and when he called before they said if he paid a lump sum then they would look at reducing it but they kept talking to him like he was stupid and kept repeating the same thing

That is what they are doing now. They have said they are taking the amended payment right after christmas.

Its over £6k they are saying the arrears are and no there was no calculations