That's where aphids hang out, so that would be my guess. They secrete a substance called 'honeydew' which attracts mold growth.

Possibly scale insect or cabbage whitefly.

Hi, looks like Brassica white blister to me, a fungal disease, Derek.

if it is scale or whitefly then an appropriate insecticide suitable for veg will sort it.
if it turns out to be the fungal blister then lift the diseased plants and either burn or put in the household bin not the compost garden bin. There is no control for the amateur gardener.

they do look like white blister to me too especially in the lower right of the leaf.

Thank you,Seaburngirl,, Derekm,, Seargram,,and Bathgate, I do get a lot of white fly around my plot. I did suspect the white fly. Why do they leave the white spot on the leaf is that the honeydew Bathgate mention if the leaf is badly infected I cut it off if not to bad I just rub them off . I have been spraying with soapy water to get rid of them.

Many thanks again John.