christmas cactus.

~They normally appear when the soil is to wet and thereofre stagnant/stale.

Its scarid fly i believe, but if you bought a well known brand of indoor plant compost, then they wouldnt of come in on it. They do just appear.
Try knocking it out of its pot and removing the new soil yo repotted it in,
Repot it with fresh compost, indoor composts better as they are permanent plants and require better compost.
Dont compact the soil eith to much around the root ball, just fill and gently firm down.
Water when the oisl begins to dry out as they can tolerate under watering and your are less likely to get these flies.

as it is a succulent plant you can let it get very dry between watering. I have had these for many years too and i tend not to water much [monthly] from april to september. then i start watering fortnightly and flower buds start to form late october in time for xmas. They are easy to propagate too. take a 'leaf off the end of a stem and press the end [from the stem] into compost. it will root and bingo new plant :o)
I have 7 at work now and they all flower at the same time.

I HAVE A LARGE FISHTANK AND ALWAYS GET A FEW OF THESE KNOCKING ABOUT.THEYVE NEVER BEAN AN OBVIOUS PROBLEM THOUGH SO I WOULDNT WORRY TO MUCH.ITS MORE LIKE THE WATER AS NICKY SAIS THERE AFTER THAN YOUR PLANT.YOU COULD PROBABLY TAKE IT OUTSIDE AND GIVE IT A LIGHT BLAST OF FLY KILLER .IM NOT WRITING THIS AGAIN BUT DIDNT MEAN TO WRITE IT IN HIGHER CASE LOL
good luck

Too loud, Man, too loud ;-)

sorry man lol

Scarid fly or fungus gnats are more annoying that harmful. If you check your soil you will probably find little thin almost translucent maggots 3-5mm long and about 1-2mm thick, which feed on the plants roots.
Unless you have a really heavy infestation there is nothing to worry about.