One must have got in and laid eggs which hatched. We had two infestations like this indoors this winter. Very unpleasant.

a mate had this and she had to get someone in to poison the others which had got under her floor, awfull but the bug man got shut of them, i hate flies and wouldnt go to her house till all gone

Great though polycarbonate panels are, they have 'open' ends which MUST be sealed with aluminium strips AND tape to be absolutely sure, otherwise all kinds of insects get inside them on the cut ends.
How do I know this? Because the roof of my conservatory looks like your greenhouse, because I didn't do what the instructions told me to do in sealing the ends of the panels as soon as they were put in place.