You need to let the soil dry between water. The flies breed in wet soil and are a sign of overwatering. After you let it dry the first time, shake off as much soil as possible and add some sandy compost, as that will help drainage and you will have no flies.

Soil is now dry. Buying sandy compost tomorrow.
Thanks for tip. Will let you know if it worked.

I bought a winter cactus from a well known Swedish furniture company last year that was infested with small black flies. I found that sandy soil and avoid overwatering got rid of them quickly - starting with the eggs.

Now we are all together...Kildermorie has nailed it.