Thanks for your question. Although each case is dealt with on its individual circumstances, typically a court will respect a parent's wishes to keep a child away from the other party's new partner if that is likely to be confusing or upsetting for the child. So yes you would have grounds and your solicitor should deal with that for you. I hope that helps. Please leave a positive rating so that I am credited for my time.
The general rule is that when the other party has contact to the child it is up to him what he does with that contact, where he goes and who he sees. You would have to persuade the court that contact with the new partner is not in the child's best interests and state why you believe that to be the case.