All green fingers out there... Small flies living in compost? Attacking plants?

Yup, i have encountered this several times, using the same cheap compost!. So, when i repotted, i went for the more expensive reliable compost, and hey presto, no more lil pests!. It did cost more money and i had to throw away a few of the plants, but some of them i saved. The lil fly things are a nuisance and hard to get rid of if you keep the plant. Before re-potting, check the roots, thoroughly but gently for any more "flies". They are the same type you get if you have some rotten potatoes you'd forgotten about(hands up, guilty of that one, lol). Try not to repot your plants to often though, so place them in a larger pot. Hope this helps.