Bluedog,When my stepfather's donkey burst after a couple of heavy frosts in Lithgow, I got him to light it late in the afternoon as he had always lit it in the mornings.
His was the external boiler and 60 gallon tank set up on a framework.
When we knew there was a frost setting, he tried to do the fire as late as possible and we had no further problems after that.
The only other solution other than replacing the tank as you did would be to try to use a pump to circulate the water around the system which I think would be a nuisance to set up.
These last 3 days have seen Temps down to Minus 3 C to Minus 10 C depending your location along the Tablelands.
I didnt know squat when I put mine in years ago and now I have almost no pressure out of the shower head so I put a small pump into the HOT line, not the best idea but it works.