CSA query

I've received a letter asking me to call them urgently - no details as to why they need to speak to me. I know my ex recently made a payment from his 1st wage in his new job, so they don't need to speak to me/contact me until his wage has been established & they can calculate what his payments will be.

I know this is about asking to come to an agreement with my ex blah blah blah & I've already told them I want my case to remain on the old CSA system, they sort things out with my ex & I get told when/how much I get. This happened a few months ago when he had a job for 6 weeks so they know my position. I've no intention of changing my position - I have my (very good) reasons & that's that.

Now, if they want to ask the same questions I'm not interested, so my query is - if I simply ignore their letter can I rely on there being no changes to this (my ex won't come to a private arrangement & has also told them this) or will my lack of response be a reason/opening/opportunity for them to close my case without my agreement?

I may send a letter telling them my position re this anyway but just wanted to know if anyone knew how the CSA are dealing with this. Thanks.