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Underneath the leaves of my broccoli, there are these white powdery marks, after i'd seen a few of these I lifted up a leaf and these little winged fly tips things flew off the spots, does anyone know what they are, will they do any harm? (I'm pretty sure it's not white fly, but could be wrong). 

Cheers

Have a look here, most likely cabbage white fly.

http://www.rhs.org.uk/advice/profiles1100/cabbage_whitefly.asp

Jerry

Thank you, that sounds exactly right

The white marks are the bit the new flies come from, any you see rub out with your thumb! You won't win but you will keep the numbers down!
;D