Spring greens with unwelcome visitors

My spring greens have some pests. They are small and a pale grey blue colour and the centres are shrivelling where the pests are. could anyone please tell me what they are and the best way to get rid of them. Thank you

 

Hi Sharon,any chance of a photo? David.

 

I think hopefully they have gone, went to the garden centre and bought some stuff that you can spray on cabbages and still eat that day, not that they are ready yet, but i guess that means its not too dangerous stuff. Last night the curl in the leaves had gone and i couldnt see anything so hopefully we are bug free now. Thanks