Actually, I paid no duty. I marked the outgoing parcel "Return for repair" and had Steve put "Returned after repair" on his customs declaration.
Maybe I was lucky. I have sent cameras for repair to the US and had to pay duty and taxes on the repair and the increased value of the item on return, which was very annoying! I do have a used Benz Micro LP-S on the way from the US, so I'll see in a few days how much duty that attracts. Even if the duty is but 5%, there is very likely to be 15% HST on top.