Fungus Gnats

These fungus gnats were all over our compost bins; covering it inside and out. They're clearly drawn to the mould in the compost. I've read that they are more a sign of a problem than an actual problem, particularly in houseplants as a sign of over-watering and roots going rotten.

They didn't seem to cause any problems on the allotment and we weren't plagued by them later in the year so don't know what happened to any eggs they lay...