When I go to the contact centre, my ex has been late on at least 10 occasions (sometimes an hour late - I only get two hours per fortnight), hasn't bothered to turn up or has just left one of my children not both. If you do go to a contact centre, they will complete a form which shows attendance records for both of you. You can then take this to the court although I doubt it will make much difference to the final outcome. Turn up on time every time if you can. They're nice people just doing a job and I think they try to remain impartial but fair - I'm lucky the staff at the contact centre I go to are good people.
Unfortunately as I said your ex can do what she likes until you get that court application in. I know it's stressful but if you want access to your children we don't have any choice. If you're struggling completing the c100 forms for the courts there's a group called PSU who will help you. They're dotted around the country as far as I know.
It's tough when the ex exerts so much control, but once you start the process of mediation/court, you will begin to feel better. Here's a link to mediation services, but you can shop around for the best deal and some solicitors also provide mediation services.
Hopefully you won't have to worry about using contact centres, but if it gets as far as court, you may find she will stop all contact until the court orders contact to resume. Some mothers realise quite quickly that they have to tow the line and court can be avoided....it's the child's right to have both parents involved in their lives and that is the courts position.
A residency order made by the court is not worth the paper it's printed on. I've taken legal advice on this and once the order is breached, the partner only needs to say that the children were unhappy and upset. The court is then obliged to undergo a review of the children's living arrangements. Social services will create a report and the children's best interests will come into play. Essentially though the initial consent order is negated. In the eyes of the law they want you to be amicable and create parenting plans.
One more point. You do not need to go through mediation before going for a consent order. You can claim exception because is your amicable it's not legally required, having said that if you're amicable then the court would argue that you don't need a consent order at all.