Honeydew on me brassicas

Use an insecticidal soap to wash off/kill the aphides. The brassicas will recover if the damage is not too severe yet.

As far as I know honeydew looks pretty much clear, I may be wrong though...

Thanks for the insecticidal soap idea, I'll give a try...

It is not a systemic pesticide so it will only kill what is there on application.  The aphids will take time to build up again, longer as the weather cools. 

Systemic pesticides work for longer but the pesticide is in the plant and therefore you when you eat it.  So best to use soap as this can be repeated more often.

You can try using companion plants to attact the aphids (systemic pesticides can then be used on those as you will not eat them).  A physical barrier like enviromesh or fleace in theory will keep them off.  But it might just keep off the preditors and one or two can quickly become millions. 

Also if you find any ant nest kill them.  The 'farm' the aphids and protect them from preditors such as ladybirds.