Hello Nige,
We are in the same boat, although I allready ordered myself a 18 mm subal extension ring. Came along this tread from some time ago:
http://wetpixel.com/forums/index.php?showt...5&hl=tokina
Udo had some nice results without an extension, If I read it before I would have tried my recent bought tokina without an extension first.
off topic:
Funny thing is I tested the tokina vs the nikon 10.5 topside on a decent tripod, my thoughts are that the tokina at 10mm isnt as wide as the nikon 10.5 (its just a little, but u would think it would be the other way around). I also found out I needed to dial in an exposure correction of about -0.66 to get the same exposure as the nikon (shutter / apperture combination after the exposure correction of -0.66 were the same as the nikon without exposure compensation).
okey, good luck,
greetzz Robin