You need to take a step back. Emotionally this is terrifying because you are fighting for the well being of your children. Against someone who you know to cause harm. For me this was the definition of stress.
However, this is just a battle, it is not the war. And even after court, things will evolve. I'm not in the UK, but France, so different thing but same abuse.
Keep doing the best you can for the children. Keep a diary of everything, calls, visits etc. In fact just plain old keep a diary; children's behaviour pre and post visits, anything and everything that might be pertinent. Type it out in a nice clear time line for the lawyer and courts.
Here we have witness statements - I'm not sure if you have that in the UK. People that know you and the kids say how you/he are as parents/people. Doctor, psychiatrist, the cleaner, my 3 closest friends, grandparents (both his and mine), your siblings, school, child minder.
Generally just work on the information you can get and give to the courts and dont worry about the rest. Much easier said than done.