Can anybody help ID these flies?

I've been trialing some environmesh over one of my raised beds to try to cut down on insect damage to my veg. However it seems that I have been trapping the insects in rather than out!

For the last few mornings I've come to this particular raised bed and there have been 30-40 of these black flies trapped in, they are fairly dopey slow flying things and a ball shaped head and thorax, they're maybe 8-10mm long and their wings fold on top of one another. I've tried to get a photo, but I can't get much closer due to the minimum focus of my camera phone.

Any ideas?

 

I was trying to get help to identify flies that suddenly appeared in my greenhouse and was told about your thread. They look very much like "mine" and after some research I think they might be St Marks flies. They usually appear around the 25th of April, hence the name. I think they are called Hawthorn flies as well. I dug some (shop bought) manure into my greenhouse borders a while ago, maybe they came from there?

 

Looks like St Mark's Flies, harmless, although grubs feed on roots but they are good pollinators.