Long so please bear with.
Dp of 18 months has a dd (3yo) with an ex girlfriend. Got contact eow and holidays through court, which took a year and 16k to achieve. She is a happy little girl and a delight to have in our family.
Ex got legal aid due to dv accusations, which she dropped in court when she was asked to give evidence. Never reported to police or social services. I have seen proof that the things she has accused him of never happened. Ex also ended relationship with her older dds df in the same circumstances but he didn't/couldn't afford to fight for her.
Ex regularly sends aggressive and accusatory letters and texts to dp accusing him of ridiculous things and threatening him to stop contact, take him to court, call the police etc. It has gotten so bad we have reported the aggressive communication to the police who took it seriously.
We got his dds medical records from the gp, as ex said she had a serious health problem (which turns out to also be untrue happily) and in the records, we have discovered that dd has a sister who, going on dates, belongs to dp and that the ex has kept the pregnancy and child secret from dp for over a year. 
We have been to a solicitor and she says that the best we can hope for is a dna test and a staggered contact after a long and expensive court battle again. I feel very let down by the 'justice' system. Does anyone have any advice that we should consider?