We have one of these! He's a fine fellow, really likes me, easy to catch, never kicks or bites, but if he sees a large, white lorry he believes he has an excuse for misbehaviour, and engages in airs above the ground. "Ooooh! I'm allowed to be barmy," as you said.
I'm planning on working him with the training in this set of videos:
http://www.youtube.com/watch?v=-m_o_po0_Ks
Watch especially Day 5, with the Dreaded Plastic Bag. Pay attention to how the trainer observes and reacts to the horse's emotional state, bringing up a minimal level of fear, and then rewarding the horse by removing the fearful object at the moment the horse relaxes.
I don't keep a large, white lorry handy, so other dreaded objects will have to do.
I am a firm believer in ground work for correcting problems that happen in the saddle. I am just a bit too old to take spills with abandon, and feel better if my bones are not suddenly rearranged. A horse properly trained on the ground, which from my perspective also includes ground driving, is a safer prospect than one trained only from the saddle.
It is my suspicion that your pony and mine both received incorrect training, in which they were punished for good behaviour and rewarded for bad. Now we have to go and undo that. Do not give up. Trainer John Lyons suggests that 200 repetitions of a lesson is a good number to keep in mind.