Thanks guys!
Momentofmadness, yep he was definitely taking the pi$$ - we were working in pairs at one point and he kept shying away from the other horse, trying to canter away in a circle, it was a hell of a job keeping him in a straight line! I got straight back on him after I was off (he was just standing there looking at me on the floor, butter wouldn't melt, I tell you

). I kept him doing walk-trot transitions after that, and he behaved (kinda) though he still kept insisting on cutting the same corner by the door. What a weirdo. I know exactly what you mean in the 'lightness' when they're being silly - it's something I've come to dread!
Packhorse, Nellie the Elephant is the one and only song to canter to

. Those exercises sound fantastic - I'll definitely give them a go and learn to relax!! Haven't been riding again yet as my back's still a bit achey so will try it next week on a saner horse!