Flies in compost.

Mods, move where you think appropriate.
Recently I re-potted some houseplants using my Sylvagrow compost. Apart from the fact that my potted-up plug plants grew some strange very slim stemmed fungi earlier on, all was ok. My OH said that my plants were covered in tiny flies. Anyone know what they are and should I spray them before bringing them back indoors?

GG, I agree, it is most likely fungus gnats.

Spraying may not treat eggs and larvae in the compost.  I would remove the house plants from their pots, rinse compost off of the roots as much as you can, and re-pot with fresh compost (from a fresh unopened bag in case the gnats have invaded your opened Sylvagrow compost).

Fungus gnats invade compost that is breaking down (fungus developing so something for gnats to eat).  They also invade compost that is too wet (which also encourages fungus).  Fresh compost kept a bit drier should help.