Do you need to tell your ex about a new relationship?

There is no single answer. There is no legal requirement to. It is your business. But if/when she finds out you are likely to be asked why you kept it secret. The practical issue is whether your children come to know about this relationship. In most cases it is sensible to keep your relationship with a new partner separate from your relationship with your children for quite a while until you know what the new relationship is going to be. Here are a few more ideas (https://www.sharedparenting.scot/wp-content/uploads/2021/02/New-Relationship-Question.pdf)n