Ooooops - i have been really dumb!! you are right, that is only what displays - it is rendering fine! - if i enable "display render tesselation" it displays fine

Thanks for the help, but one other thing, the planar surface is a trimmed plane. I want to intersect something into the plane, then cut out the bit i intersected - is there any way i can do this? if i select the trim tool, it will not let me select that plane

Thanks again