New to the CSA

Hi,
I am soon to be assesed by the CSA for payments for my daughter.
This will be with a view of direct payments to my ex, rather than the CSA collecting the money from me.

I have for the past 3 yrs paid exactly what the online CSA calculator has told me.
I have never hidden money or with held money.

My qestions are:
1. What happens next?
2. Do I get a letter or phonecall from them?
3. How long do they take?
4. Do I carry paying as I have been?
5. Will they take what I am currently paying into account?
6. Or will they ignore my current payment and say I have arrears?

My new partner and I have a new baby, I see my other daughter every other weekend for two nights and every wednesday night.

This has come about due to my ex not trusting what I should be paying her.
I think she thinks I have had a pay rise and not told her.
She is also bitter about me reducing the payments since the birth of my new daughter.

I have always been 100% down the line, never paid less and on occasions paid more than the calculator said!

This is all very stressful!