In reply to andy:
Agree. Haven't done Feb half term, but have hit last week of French hols in March, and it can be bad. One thing to bear in mind is that local registered minibuses are allowed to use the old back road on the other side of the river (the Police stop bigger and non-local traffic from using it) and can so bypass a lot of the jams. So if you can find a transfer company who use buses that meet that criteria, you cut out some of the jams, but from what I remember it's not all of them - there may well be a lengthy jam before they can cut off the main road. (We used a Coolbus this year who did this and explained the rules, and have also been rallied along it by other companies in the past)
Haven't done the train thing, but friends did it with two really small kids in tow and it was apparently fine.