Support for children

Hello I am a mother of soon to be 4 children but 3 of my children were born in berlin sana klinikum to be precise and they all have German passport due to their father being a German citizen. He works and live in germany (berlin) as an assistant team lead for the German red Cross. I being their mother I am live in the uk with the children. We are still legally married but he doesn't pay any child support or assist me financially with the children and am aware he might be claiming child benefits for the children too. I would like to know how I can get support or make him start supporting his children financially. Who should I contact or speak with for guidance about this issue? Any form for assistance would be greatly appreciated. Thank you and i look forward to reading your reply.