I have just come back from a weekend away and seen your question.I had to get the insect book out as I also found loads of these in the garden last week and thought they might be something nasty, like you have never seen them before, at least we shouldn't have any trouble with the greenflys this year
If you have lots of these in your garden, it is both good and bad.
Bad, because it means there must be a plentiful supply of food for them - i.e. aphids,
and good, because they will devour the evil little green beasties with a vengeance and hopefully eradicate them.
You can get a 'bug box' which is a bit like a bird box, but made with rows of bamboo cane cut up, and the bugs hibernate or nest in them and when they come out they eat up all the greenfly etc.