the tiny flies might be fungus gnats or fruit flies and are often a problem if you over water your plants and they're constantly damp (in the house). You can buy special houseplant compost, but if you're overwatering your plants, you need to correct that to help with the problem. For most houseplants, the top of the compost should be dry to the touch, but not shrunken from the sides of the pot, when they're ready for more water. When you do water, water thoroughly, then tip away any water left in any outer pot or tray after 30 minutes. If you still have a big problem, there are various stick insecticide treatments available for houseplants.
Wickes own multi purpose compost is pretty good - I bought 6 different brands last year, and that turned out to be the best - most were full of bits of wood, wood fibre and other assorted rubbish, and certainly not suitable for houseplants.