We're in Norfolk now and we have ash dieback up here too ... we had two large ash trees in this garden ... one had a bit of dieback and the tree man said it wasn't healthy so he felled it for us about three years ago, but he said the other one looked fine to him ... it's certainly still growing well and looking healthy ... according to the experts there appear to be some strains that are more resistant to the dieback disease, so if your tree is doing well and is a female producing seeds it might be a good idea to look out for saplings that you could utilise for your copse. If not, maybe consult the Woodland Trust who are happy to advise
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https://www.woodlandtrust.org.uk/plant-trees/advice/
Keep us informed ... we love a project ... before and after pics are great too
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.