Ok. I admit it. I was wrong and am a confirmed idiot.
After a long and bracing walk this morning, the cogs at the back of the Aubrey brain began to turn again and a PDM occurred. The final stages of the gridfill and a very satisfying endgame came quickly thereafter, accompanied by a bao bun stuffed with Korean fried chicken that was - almost - equally satisfying.
Thank you, Botox; and apologies for my earlier churlishness.