Edit - England based

My partner has had to get a Non Mol and occupation order against her ex Husband due to violence against her and her daughter. The Ex husband is a father toto there shared son (2 years old), she has not limited contact with his son and he has him 2-3 nights a week.

The Ex husband is now threatening me with violence (through her on message so have proof). This breaks his Non mol with her but she is loath to report it as she instigated the conversation as it was him introducing his new partner.

Obviously I see it as a matter of time until he actually does attack me, I will not instigated or retaliate but I would like the police to be aware of these threats now due to his history of violence, ready for when he does do something. How do I report this?