Pain has been ruled out - this evening one of his field mates was being ridden in the arena so I decided to hop on and see how he would be with another horse in the arena. He was much better - no bad behaviour. He did pitch a tiny fit when first asked for the trot but he got over it.
I know the general attitude towards gate sour issues is to work him where the gate is, only allow him to rest/dismount at the other end of the arena - I already do these things. The footing in the arena hasn't been very great to work him very hard near the gate, but when he acts up I have been working him harder and then allowing him to walk out calmly only when moving away from the gate.
However, what I am really looking for here is confidence building exercises. It is clear to me that when he is alone, he isn't confident - I know there have got to be some people in here experienced with this (maybe should been in the NH section? If so, feel free to move it admin).
Thanks in advance.