I hope that your back recovers soon, and that toddler calms down 
In terms of 101, it probably wouldn't be my starting point if he refuses to leave - I would be clear first of all that any progress with things relies on him respecting you and what you ask, which is right now, that he leave. This does not mean that you won't talk to him about things, it means that you won't talk to him about things right now, but when you are ready.
Then repeat 'I am asking you to leave' as necessary; without commenting or responding to what he says. You will need to discuss issues with him at some point, but right now, you are asking for space and it is better all around if he respects that.
I can't advise on the right to stay issue, I am afraid, you need a solicitor for that. If you feel physically under threat, then he should not be coming in the first place, I think - and if you begin to feel physically under threat when he is there, then yes, you should call 101. But that apart, I would try standing your own ground first (with your phone in your pocket or otherwise accessible). I personally think the fact that there are no locks on the rooms, and he is not leaving when you are asking, would be sufficient to suggest that you feel physically under threat.
I called 101 for advice about five months in, but at that stage, I had a solicitor involved, and he had been written to three times about the situation and was ignoring the requests not to come to the house, to the stage that it did unquestionably constitute harrassment. Acceptable contact arrangements for the children had been provided; as had clear plans about who lived where and finances. My solicitor was clear that if I said I would call the police, I must follow through; but I did also call 101 in advance and ask them how and if they would be prepared to respond. To which they said, yes.
In short, the best people to advise on calling 101 are the 101 team, and you are free to call them at any time, and not leave yourself in a position where you are trying to make a judgement call at XX time in the evening.
In terms of what he says to the children, that is unfortunately up to him. You have asked to separate based on the fact that you do not wish your behaviour to be controlled by him; as such, you have to accept his freedom to offer the explanations that he wishes, until such time you have evidence that his comments or behaviour with the children is harmful to their well-being.
Being kind, by the way, is subjective - if you tell him this is what you have said, then it needs some kind of clarity what kindness is - it would be easy for him to say 'but I am being kind, I am here and I want to stay with you and make sure that everything is fine....' - whereas what you really mean is respect for each other's needs and wishes.
Anyway, hang in there, you will find a way through 