Paying csa after child turns 16yrs old

I got divorced a couple of years ago, i have 2 boys which are now 16 & 19 yrs old, when i left the family home, i paid the mortgage, her phone bill & an mutual agreement of £600 a month maintenence. which i paid through my online banking. when my oldest was 16, i told my ex wife months before i was dropping the £600 to £300, she said ok as long as you pay for the oldest one, which i said no problem he`s my son, when the first £300 went into her bank i got a drunken voicemail saying ” get that other £300 in my bank or im going to CSA” i called her next day and told her not to threaten me & go where she likes.

So CSA rang me up & ask me all these questions, i explained im not a father who doesn’t want to pay, i want to pay what is right, in the end i told them to go to my employer as im sick of all this questions, so i got assessed & paid what i had to pay taken from my salary.

Now my youngest son was 16 in may last year & continued to have CSA taken from my wage until november, then get a letter saying it should of stopped in September, so i guess im 2 months advance. now my lad goes to college to be a joiner & csa have told us 2 things, its a new claim now, so we are re assessing you & want your november & Decembers wage, today my new wife called them & they said its not a new claim were just re accessing you. so now im worried that i get assessed as my dec wages had overtime on & they think i get that every month.