If you and the child's other parent cannot agree on maintenance, or if the other parent is recalcitrant, you can submit an application to the UK Child Maintenance Service. This is a publicly funded service available to UK residents. There is also the Child Support Agency which is available for especially difficult cases. They have a cadre of enforcers with foreign contacts who can be ruthless.
The 'best practices' advice is to examine the rules and see if you qualify to submit an application. The starting page for you is Use the Child Maintenance Service or Child Support Agency (CSA).
Take note that these agencies have been hit hard by budget cuts and most of their services will be ended in 2017, so you need to hurry. The answer provided by Dorothyanswer provided by Dorothy (to whom a tip of the hat), explains that a solicitor can be helpful. This is great advice and should be heeded with alacrity.