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Oxford study proves heat pumps triumph over fossil fuels in the cold
(www.nationalobserver.com)
This is a most excellent place for technology news and articles.
Under the conditions you describe, a hybrid setup will work best.
In a hybrid setup, the heat pump is providing most heat when it isn't very cold, while the traditional heat source is providing heat on really cold days.
A few years ago, the temperature where the system would switch was 5 C. Nowadays it's more like -10 C. As heat pumps get better, hybrid loses territory, so you could also just wait a few years and then switch.
Hybrid gives best of both worlds at the cost of added complexity.
If you have A/C then the cheap way to do hybrid is to keep the traditional heating system but use the A/C in heating mode on mild days.