It seems a shame to involve the courts after 5 years. Good contact for children relies on parents working together . Going to court tends to leave parents feeling resentful and resistant making working together difficult. Even if the mother is dismissive a few sessions of
mediation may help diffuse the situation so constructive progress can be made.
In any event before you apply to court you (or a solicitor on your behalf) needs to write formally and ask the mother to sign a Parental Responsibility Agreement. The letter can then be used in court if necessary as evidence that you have been reasonable and tried to resolve the matter between yourselves before applying to court.
If your ex refuses you would need to apply to court asking for a Parental Responsibility Order to be imposed. There is then a requirement in most cases that the applicant to court proceedings sets up a
mediation Information & Assessment Meeting with an approved mediator before the court will hear a case.