What's eating my sweet pots?

I've had sweet pots in the ground for a couple of months now under a small poly tunnel, but they outgrew the poly tunnel and so they've been exposed for the past week or so.

I checked on them yesterday to find some holes in the leaves, any idea what it is? Doesn't look like slug damage to me, holes are too small. There was low flying gnatty-type flies hovering around it when I was there about 8pm yesterday.

Thanks for any advice