The C100 is really only if the ex won;t agree.
In your situation I would get a solicitor to draw up a contact agreement which he would then send to the court and ask for this to be converted to a legally binding
Consent Order.
I expect he will need to quote your court case number from before.
A C100 is around £250 which shouldn''t be much different from what a solicitor would cost.
On the other hand I''m wondering if you could by-pass a solicitor and draw up your own contact agreement and send this to court.
You need to try and find out.