It sounds like your ex-partner has stopped contact as a knee jerk reaction to the reduction in maintenance for the children. She is angry and is punishing.you. It does not seem as if she is considering the effect this will have on the children, given they are accustomed to spending two or three nights a week with you.
If your ex-partner cannot be pursuaded to reinstate contact, you may apply to the court for a defined contact order. You will need to make the application to the County Court or Family Proceedings Court local to where the children are living. You will need to complete a Form C100 and pay a fee of £200. The forms can be downloaded from the Court Service website - http://www.hmcourts-servvice.gov.uk/ The court office should also be able to supply you with the form and are generally quite helpful in providing information, but not advice.
The fact that your ex-partner is in dispute with you over the maintenance you are paying will not be recognised by the court as a justification for stopping contact. The court is only concerned with what the best interests of the children are and if there are established contact arrangements, your ex-partner will need to explain why she believes it is in the children's interests to change these.
You also like to consider mediation with your ex-partner. There is a general expecation that parents will try and resolve disputes about the arrangements for their children by negotiation and mediation, with the courts being a last resort. If you are unfamiliar with what mediation involves, there is lots of information on the Web. Try the Family Mediaiton Website -www.thefma.co.uk or just google ''family mediation''.
I hope this is of some help.
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