Fungus Gnats

I have a massive outbreak of these pests amongst my seedlings this year, despite keeping the compost frighteningly dry and only watering from below. 
Given that each female can lay 200 eggs I'm wondering if I should try and repot as many as possible to try and limit the damage?
I also have some pyrethrum based insecticide (for cat fleas etc) which I'm contemplating diluting and using as a drench or spray. Although organic, I'm not happy about doing that, but at the moment all chillies,  peppers  & aubergines are infested.
Nightmare 
Any thoughts would be much appreciated.