[user104658]

Originally Posted by Niamh.

Or check that you don't have some old fruit or veg rotting away somewhere sneaky

We had a massive problem with tiny flies at one point for over a month, could NOT get rid of them, no matter how often I removed and cleaned the bins... couldn't find a source...

...eventually discovered with it was. A banana had fallen down the back of the microwave and had been there for months .