You have my sympathy. I have personal experience of this and I've seen how destructive some parents can be.
Does the contact order express a collect/delivery point? It's quite usual for them to do so. Otherwise, the mother could just clear off to Scotland and say "well he's available, come and get him". So you COULD apply for a variation to the order to express a particular collect/delivery point.
I wouldn't recommend you go back to Court over one missed day - albeit I know it was an important day. Is there any chance you could agree with your ex another day when you could have "Christmas" with your boy?
If this is a regular pattern, you can apply to the Court, but as has already been pointed out (and as I know from bitter experience) there are no amount of court orders in the land that can make her do something she is dead set against. Even if you had a penal notice attached to the order, they are rarely enforced.
Courts like to look forwards rather than backwards so try and think of a plan for next year so that you can see him. Try and also see it from her point of view - she went to stay with her parents on Christmas day, she would have had the same transport problems in reverse.
If at ALL possible, try and communicate sensibly with her. I appreciate it might be difficult but bite your lip for the sake of your boy. (And trust me, I know that that is not easy - I am stuck in the middle of two parents who could gladly kill each other at any given moment!)