Hi mrcharly-YHT
Bars are 5mm lower than the saddle, saddle is level.
I've judged balance by being able to raise my hands off the bars, both on the road and on a turbo trainer, which also implies from I've read my core is reasonably strong, though I've been doing the plank daily @ 3x 60, with the intention of increasing duration over the coming weeks to increase its strength.
I'm coming to the conclusion that my upper body, particularly my arms are not as strong as they should be, which possibly explains part of what is happening. My first thoughts were that it largely happened when a lot of climbing was involved, eg honking. But recently tried staying seated as long as possible, then walking, however it didn't alter anything.