Update - I just changed the bedding. He was a little star - I tempted him into a mug with a treat, then put him in his playpen with a few toys while I cleared out the cage, and he was pretty much unfazed
I've filled it up with just hemp on one side, and a thick layer of the recycled paper bedding on the side where his house is, because it was the only other bedding material I had to hand - I'll try to get Carefresh or Kaytee for the next change.
When I put him back in, he sniffed all around, then went to his food bowl (which I'd refilled since I chucked out his hoard with the old bedding). He was fine while exploring the cage, but sneezed a couple of times while pouching his food. Could the food be the issue? It's Harry Hamster mix (not sure what he was eating at the pet shop but I can check next time I'm there)?