I think the only advice with this problem is to wait and hope for the best. It's what I'm currently doing with mine but there is nothing practical we can do. If they show new signs of life then it will be pruned down to just above the growth, if it doesn't then at some point it will be taken out. They are borderline hardy in a good year so there will always be a risk, should we get a harsh winter.
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They are borderline hardy in a good year so there will always be a risk, should we get a harsh winter.