Whatever you decide to do, keep in contact with your boys/girls and they will come back to you. Eventually !
Very much this.
I was advised against spending a fortune getting court orders as it's still easy for the mother to avoid you getting the contact if that's what she intends to do. The courts are very reluctant to put a mother in prison as they need to consider the child's welfare above all.
My ex did all the mental poisoning of my kids against me and my new partner (who wasn't involved at the time of the split), but we kept being ourselves and letting the kids know we cared. I even had a run in with the school over a couple of issues when they were less than impartial and completely out of order.
Slowly but surely, my son saw through the lies and now has a better relationship with me than his mum. My daughter has taken longer but has finally thrown most of the baggage she was handed by her mum to one side and we get on much better again.
My main advice is to make sure you never involve the kids in any of the nonsense going on between their parents if at all possible. Just continue to act in a loving way towards them and hopefully they will see the lies for what they are.
I don't envy anyone going through what we did, but I'm delighted to say that I can finally see a situation where the future is bright once again.