brassicas disease help

hi all, i seem to have some sort of pest or disease on a couple of my cabbages, i also had it on a few broccoli when they were smaller.
its like small patches of very small grey eggs mixed and surrounded with a grey dust and seems to be powdery. iv looked in the veg expert book and at first thought mealy aphid or cabbage white fly, however iv not seen any adults or anything moving around in the patches on the leaves, the plants dont seem to be effected at all.

any ideas?

cheers binner

They probably are mealy aphids. When it is cold they move very slowly. Look at some with a hand lens. Do as peterjf says.