OK, well in case anyone else gets this problem.
I went to the quack who agreed it was the bone between the talus and ankle bones (cuboid), that there was inflammation to all the soft tissues. But I was was surprised that he didn't decide to do the standard treatment which is a simple manipulation called the Cuboid Whip/Squeeze, which pops it back. I was just told to sit on my arse for a whole week and take ibuprofen. Anyway, no riding or running and avoid walking too.
So I was doing some non-load bearing flexing/stretching – rotation and flexion up and down etc. I was flexing ankle and toes down and there was a nice "pop" and although still sore, the foot suddenly feels noticeably more comfortable. So I think I just managed the Cuboid Squeeze by accident!
Here's hoping it calms down in a few days and I can get back to riding.