All broody hens should leave their nest every day to eat, drink, and poop, maybe even take time to take a dust bath. I've seen a hen leave her nest twice a day for over an hour each time in pretty warm weather. I've seen a hen leave her nest once a day for fifteen minutes in colder weather. Most I never see off of the nest at all but I know they are getting off, they don't poop in their nests,.
Before they go broody they store up excess fat. That fat is mostly what they live off of while broody so they can stay on the nest instead of having to spend so much time looking for food. It's good for them to eat something, that fat will last longer, but if you notice her losing weight do not panic. That is fat put there for that purpose.
it's only 5c out and the eggs were stone cold when I found she had gone back to another nesting box for an hour!
Giving it another week or so and them candling is a good idea. I think you will be pleasantly surprised. Many of us have stories like this with a power outage with an incubator or a hen doing what yours did, going back to the wrong nest. Something like this is not good, but most of us have good outcomes with those stories. I had one that I think was off of the nest for a few hours before I saw what was going on and put her back on the right nest. The eggs were ice cold to the touch but she still hatched 11 chicks out of 11 eggs.
What likely happened is that another hen was on her nest when she came back from her daily constitutional and she went to another nest. Most of my broody hens would still go to the correct nest. The girls are usually willing to share, broody or not, but my nests are fairly large 16" x 16". Did you mark the eggs so you know which ones belong? You should be removing any that don't belong. It makes life easier.
Good luck and let us now what happens went you candle.