I thought that as there was no reason for the problem then it would ease once I was out of the wind, but no. By the time I had picked out his feet (and he is a good boy for that) I couldn't feel the leg at all ... so decided to call it quits.
I went in and told Toni I wouldn't be riding; and we chatted for a bit. they have a nice extension in their kitchen, with a bit of wall that is just "lean-against" height.
I then went out and fed "sir" and rode him back to his field - I did put on his bridle, but not his saddle. So all the work he did was a gentle stroll with me, but no tight girth!
So - although my back was hurting, it was a type of morning that DJ could cope with quite well!!
My leg is still numb, but that hopefully will re-settle within a couple of days.