Nice work but you beat me to an instructable! I made this about a month back, almost identical design :). Works great! I used pieces of 90mm downpipe with a slit out of it for the NFT (gravel) holder, which seems to contain the roots. The whole thing is about 3 metres long. Have got about 8 lettuce plants, peas, beans, and a lot of tomato plants which haven't started producing yet. I haven't had a problem with the roots blocking the flow of the water. The growth really sped up when I reduced the PH to 6 from about 8 and backed the fertilizer off a bit.