Matt,
Popped over to your blog.to answer your question,(with out sounding too preachy
)I've built couple of these now and moved them on,with out any effort,which brings me to my point. I use n gauge hedges and trees,don't be tempted to buy war gaming 15mm trees etc,as there generally the same products at a higher price.(a quick trawl of eBay and you'll see what I mean)
But the best way to do the box hedging for formal gardens is simply use the green scouring pads you can buy in packs of A5 and A6 sheets in the cleaning isles. They cut very easily with scissors ,and the odd match,cocktail stick,etc to raise them slightly off the ground and paint and texture to taste ,this method means you can produce really intricate patterns for next to nothing,and adding the odd bottle brush trees in the corners is a great way to give additional strength .
Mark.