Carrot leaf eater

Hello all I'm growing carrots but something keeps eating the tops of them - virtually the moment the seedling starts to grow it's gone. What I can't work is what - I use slug pellets and to be honest haven't seen any slugs and the whole of the vegetable plot is in a cage. so it can't be birds. I've checked for caterpillars at night and can't see any.
I'm not sure if this is related but all my brassicas (ie broccoli ,cauliflower, cabbage and swede) a have big chunks taken from the leaves. I've not come across this before - previous years have been fine although this is the first year I've tried to grow cabbage - I'm not sure if that has anything to do with it? Any help/ideas greatly appreciated Regards Helen

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