For the greenhouse you can get yellow cards to hang up that attracts the white fly or you can try using Bug Clear and spray in the bags tie up and hope it does the job.

Or you can simply ignore them...

If they are coming out of the compost, they are probably fungus gnats. They are often, but not always, a sign of overwatering, since the larvae feed on fungus growing in the soil. Organic mosquito control granules containing Bacillus thuringiensis israelensis usually work, just sprinkle them on the soil and water in. If for some reason they don't go away, soak the soil with a solution of carbaryl, or use Provado granules--though not on anything edible!