All - very grateful for the posts, which were uniformly insightful and well intended, so no quibbling please. Cubewheel's comment pretty much nailed it - first hard effort in the drops had the longer leg working harder.

The last fit followed a few years of me making minor adjustments with the cumulative effect of making things worse. So short term, although I'm pretty sure there would be no harm in lowering the saddle a touch, I think I'll move the SPD set-up down to the trainer and add the shim.

When the new bike comes, I'll head back to visit my favorite PT.