Thanks for the replies ...
Joanie ... I made some boxes last year out of old pallets and did like you said and left plenty of gap with mesh covering the holes but no matter how much I watered the water just seemed to pour out everywhere, not just out of the drain holes but out of all the seams too and leave dry areas in the soil, this time I have hardly any gaps at all and the water takes a while to drain out but seems to wet out the soil better, I'll see how it go's, if need be I'll drill some more holes in the bottom.
Alasdair ... I would have thought any chemicals leaching into the soil would be easily flushed out but as you have said it appears that the chemicals can actually bind to and hang on to the soil.
El Gringo ... I do tend to use coarse gritty mix so this could be beneficial in flushing as you suggested.
I also just noticed an interesting post by matt down below in the Similiar threads list about grow boxes thats very interesting ...
http://forum.bonsaitalk.com/showthread.php?t=1803