Fungus gnats!

I love the internet.  I googled 'tiny hopping black fly', and within a few seconds had found its true name.  My uninvited guests are fungus gnats.  I can imagine this is what the lady in the Rumplestiltskin fairy tale felt like after she'd discovered him shouting his name in the woods. 

They aren't actually directly harmful to the plant, they just infest the earth with their eggs.  I like to give my plants a clean living environment, and egg infested earth doesn't sound like it will benefit the healthy growth vibe in my happy orange propagator, so I'm glad I've got the cayenne spray. 

Here's the useful article I found about fungus gnats:

http://whatdoino-steve.blogspot.co.uk/2009/11/tiny-black-bugs-fruit-flies-or-fungus.html