You could put a root barrier in place but I would just paint glyphosate on any bits that sneak under the fence and cut off any branches that home over or through the fence.
Pretty much the same as if you were regularly clipping a hedge to keep it neat.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.