Hi Andy: Thanks for the reply. The reason I asked is that I intend to use Lee filters. They produce a special set of ND Grads for use with wide-angle lenses. Their size is 130 x 170 x 3mm. However they do not produce a filter holder for them and state they are to be used on a Cokin X-Pro holder.
see:
http://www.robertwhite.co.uk/product...2244&PT_ID=549
However, having said that: You say you are using the Cokin 'P' series holder (no hood) do you not get any vignetting when shooting at 28, 24mm or wider? Do you use the standard adapter ring or the wide-angle adaptor ring to connect the holder?
I passed through your neck of the woods last Thursday on my way to Bodnant Garden to try out my new 24-70 2.8G by the way. Looks very beautiful.
Cheers Hal