Turned out to be a hole in both the idea and the run. Didn’t spot it until going to check up on him. Too small a gap for a hen to squeeze through but a breeze for an adventurous hamster. Feared it might mean a trip to the pet shop for a lookalike, but spotted him in the flower bed under the crab apple tree.
G said later he thought I was weeding (as if). I was rugby tackling a hamster in amongst the lobelia. He’s safely back in custody now.