Last year I had quite flies in my new compost in my herb box

Hmmm. Neither whitefly nor greenfly will live in bare compost. Are there living herbs in the box now, Jewels? Is it possible that they are coming off the leaves, rather than the compost? If not, are the flies a deep, metallic green? Did the white ones actually fly, or just jump? You can tell if they are flying, because at least a few will do circles and Immelmans in mid-air, while the jumpers just obey gravity.