Children maintenance to ex

Hi,
Information: 2 kids stay with mother (11 & 12 years old), father provides significant maintenance (mutual) but still less than what CSA would calculate. Mother and father remarried and have one more child each. Mother divorced again so also keeping this 3rd child (from another father). Mother doesn’t know details of father’s new job or address.
Questions:
1. If mother contacts CSA for child maintenance (for 2 kids from their marriage), what information would CSA ask to start the process and track father?
2. Post Brexit, will there be any changes in rule for child maintenance calculation?
Thank you in advance.

1. As much information as possible: name, date of birth, national insurance number, last known address, last known employer, car details etc. They can usually find most people quite quickly due to number of organisations they can get information from and information they can legally demand.

2. Not directly as a result of Brexit but obviously there could be changes in the future as these things always evolve over time.