His doctor called late yesterday as we were going into London. He had sent Ollie's test results to a specialist in Washington state, who called back with the diagnosis that Ollie had had a couple of minor strokes.
It's not as bad as it sounds. Small strokes can be caused by many reasons, most of them transient, and his behaviour hasn't changed at all, so it doesn't appear they've caused any damage. We'll start him on medication to help prevent any recurrences.
In the scheme of things, this is a better result than a spinal blow-out or epilepsy. He's also fully back to his previous self. (I just now took the attached photo). I think the spinal tap took more out of him than we thought.
As I posted before, Sooz and I are going to try to relax now. Ollie's had better health care than 99% of the people on this planet and we've done all we can for him. He's still the dog we love so much, only a bit slower. We're happy to live with that.
UPDATE: Two things. First, I should have mentioned that dog's brains can rewire themselves much more efficiently than human's after a stroke. And, a Cardigan Corgi won best in group for the first time ever at Crufts, which is the biggest dog show in the world. This is a great achievement for Ollie's breed!