The message
from Ben Edgington contains these words:

" Jeanne Stockdale" writes:
I have got the same problem - noticed it in previous years but never as
badly as now. There is an earlier posting entitled "greyflies" which
deals
with this. Have just got some"Bug Free" to spray over the sprouts - only
trouble is it is too windy at the moment to spray

Thanks for that, Jeanne. I somehow missed that thread over the
weekend. Have
now read it, and it looks like I will have to get some insecticide.

Before you do, there are alternatives. Brassicas can withstand salty
water, so try spraying with that; maybe with a squiret of washing up
liquid. Or, if there's enough area of healthy unaffected leaf, just tear
out the aphidy patches and dispose of them. Or, hang some nutfeeders on
canes to encourage bluetits. I have done this to remove really huge
infestations of aphids on trees, the birds clear them in the space of a
day or so.

Janet.