Maggots between the lid, top of the bin and around the filter vent.

[Aleksis Zarins]

I have suddenly noticed small maggots around the top of my bin and the lid edge. Also in the vent at the top.
I cleaned them all off and cleaned the edge of the bin. Difficult to clean the vent/filter area.
I then watched quite large flies with greeny/blue backs squeeze their bottoms in-between the lid and top of the bin and within 5 mins there was atlas 50 small eggs!!
Anyone know how I can stop this?