Anyone have flies/ bluebottles appear in the house?

having trouble with house flies and some bluebottles past couple weeks. some days we could catch 6 or 7, some days 1 or 2, (usually by Henry the hoover). just don't know where they are coming from. I've checked anywhere I can for dead things, no funny smells around either. I've sanitised sinks and drains best I can. I'm starting to go crazy, and my pet lab is going insane. Recently my rotten decking was lifted, and there's a broken drain cover right outside kitchen window, wondering if that could have something in it. (don't fancy putting my hand down there)!! edit... can flies or maggots come up through an outside drain?