Alasdair

What you need to do is sever the leaders at the height you desire you're bonsai to be. Doing this causes the plant to put it's energy into growing the branches further down the trunk. The longer and thicker these lower branches are, the thicker the trunk gets to support them. If you don't cut the top back all you will get is a very tall, relativly thin pole.

Al

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I can feel another "I wish that was my tree" moment coming on...
Currently studying BSc Plant Biology at the Universty of Wales, Aberystwyth