Flies in leek tray

Help please. I started off a tray of leeks (potting soil in a tray in which I sowed some leek seeds) and the seeds came up a couple of  weeks ago. They are growing wellish (a bit  spindly) but the couple of tiny flies I noticed a few days after sowing have now multiplied quite a lot. Is there anyway of getting rid of them (the tray is in my mezzanine)?

I also think I should start putting the tray outside for an hour or so every day (although it's 7 degrees centigrade outdoors at the moment) to help them to get stronger.

Thanks a lot for your reply. Yes, that's probably what they are.

Anton