I can’t imagine flies consider anything to be repellent. I think Lavender is a general insect repellent and theres a list of natural insect repellents here – although you’ll see few of them mention flies.
If the house is new / new to you and has been unheated for a while then putting the heating on might have caused hibernating bugs to think its spring.
I stayed in a self catering cottage out of season a few years back and it hadn’t been booked out for a while. Switched the heating on and with a few hours things started to get a bit Amityville as dozens and dozens of flies started appearing. It was fine after a day or two though.
Planting stuff outside here (tomatoes etc) and basil was recommended to keep insects away. Seems to have worked as despite being next door to a swamp (sorry mashland habitat) we’ve had good tomatoes, no pests either. Strawberries have been fine too, but pretty poor size it seems.