You might find it helpful to post in the MN legal section.
And to look at this website childlawadvice.org.uk/information-pages/residence/
Organising a mediation session is easy. It's just one hour, and some mediation centres reduce the charge according to people's income.
It's possible to represent yourself in court. The difficult parts are:
1- identifying the correct forms (ask in the MN legal section or google or ring a court or look on Childlawadvice website listed above)
2 - working out what you want to ask the court to order. Some tips below.
Actually going to court and talking in front of the judge is quite easy (but be prepared for the other side to say things that are not true. Will need to stay calm, and simply say to the judge - what was said was not true.)
And going back to court to enforce an order is not difficult, but you need to have some reliable evidence to prove that other side breached the order. This can be tricky if it's a case of 'he says/she says'. Something in writing from other side stating why they didn't do what they were supposed to do would be helpful. And generally you would warn on the first occasion, and only go back to court after it had happened 2-3 times.
Some tips on what you might want to include in the Residence Order:
Obviously you will have a clear idea of how much time your DP and you spend with his DCs in a typical week/fortnight. So the order might say: children to live with Mum except as provided below.
Children to be in Dad's care: Saturday 9am to Sunday 9am every weekend [this is just an example]
It is also a good idea to get the judge to order what is to happen with as many potentially contentious issues as possible. Eg holiday time (how many weeks per year with Dad, how many consecutive nights and at what ages), Christmas/Easter (where do children spend Christmas Day/Boxing day every two years, where do children spend Good Friday to Good Monday every two years); when holidays are to be discussed/agreed (4 months in advance?), holiday travel (with consent of other party only? what information to be given to other parent about flights and accommodation), what changes will there be to residence arrangements when the children are a bit older (whole weekends? evening meal during the week) what changes will there be to residence arrangements when the children reach school age, pick ups/ drop offs (by whom, where), contact with other members of his family (including you) (the Order could expressly state that this is fine), how variations to the normal routine are to be handled (occasional variations only and by mutual agreement?), communication between adults (by email only except in emergency; business-like only), communication with children (by FaceTime once/twice per week to be organised by mum at what time? Between 5.30 and 7pm?).
Like other PPs have said, the long term benefits of getting this sorted will greatly outweigh the short term pain and difficulty.