Update. This morning I drove to a nearby Botanic Garden to meet a friend for lunch. The car was still at about 93%, but since the charger was free to ChargePlace Scotland members I thought I'd just top up. So the car was on 100% when I left.
It moved to 99% very quickly, and I was seeing 1% regeneration. The road went gently downhill for a bit, so I just took my foot off and let the car coast. The display went back to 100% after a couple of hundred yards of this! So it's able to regenerate at least to some extent even at a very high SoC.
As soon as I put my foot back on the accelerator it flipped back to 99%.