I didn't do sweet trees Phleb, I did flowers ones when I was floristing :)
I think that's a lot you have paid for the mechanics, you can get a 2400mm length of dowel from Wickes for £2.88, that would get you about 6 trees.
http://www.wickes.co.uk/Wickes-12mm-Light-Hardwood-Dowel-FB153-2400mm/p/128317
Is some of the stuff good for more trees? The initial outlay can be pricey but the next tree will have more profit as you have most of the stuff. I was having a think about your oasis as it really is a pricey waste as you don't need it for keeping flowers alive, I found these polystyrene balls which are much cheaper and much better suited for your purpose.
http://www.fredaldous.co.uk/polystyrene-balls.html
Check Ikea for plant pots (they look much nicer than those buckets), about £2 each for Papaja ones and they seem to come in a few colours. Try getting ribbon from a £ shop.
If I think of anything else I'll let you know.