He's just had a lovely bit of gentle back treatment and is feeling all relaxed - so the plan is to go back to the beginning and, by process of elimination, see if we can find out what's bugging him.
Today he started with a head collar and a bit of long-reining and, as you can see, he behaved impeccably. No problem there.
Next we'll try him with a bridle, then a saddle. If none of that bothers him, Linzi will be up in the saddle and hopefully, by then, we will know what the trouble is.