Fungus gnats?

I read somewhere (could have been on here) to put a small dish of wine or beer near the plantpot for the adults to fall into dead drunk. I tried that and found half-a-dozen of them in it each morning. Another tip was to catch the larvae from the soil, by laying slices of raw potato around the plant. The theory was that they come to the surface onto the potato, which can then be thrown out and a fresh slice put down. That seemed to have no effect on mine, but perhaps the adults hadn't had time to produce offspring before they went on the booze?

You are not by any chance in the habit of watering your plants with left-over tea or even milk are you? Fungus flies love the smell and will inevitably appear if you do this. They are also attracted to plants which are over watered where the surface of the soil is always damp so your idea of gravel over the top is an excellent preventative.