Update: Took the cover off the unit and found the reversing valve. Indeed it was sticking post-defrost. I gave it a little tap with the handle of screwdriver and, hey presto, it changed position. So it’s clearly sticking at low temperatures. Before next defrost I put a little hot-water bottle wrapped up next to it… and it changed position fine after the defrost. Soooo… while waiting for the Samsung software update to put more energy behind the valve, I’ll see if I can arrange a little 10 Watt heating pad around the valve to keep us going… Thanks everyone for your help.
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