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Hi all. I've recently acquired a healthy 4foot dwarf Scots pine. I believe it's around 10yrs old perhaps less.

It's my first Pine (apart from some young seedlings i have cultuvated).
I would like to begin developing it towards a bonsai. However the top half is like a Xmas tree, tall and boring. But the lower half is interesting, with the potential to be a sort of twin trunk, with taper.

I want to know if I could remove the top half without severely damaging the tree? If this is possible, when is the best time?
I've attached 2 images to help. And also I live in the North of England.
Any help much appreciated.