I live in the northwest UK. On Christmas Eve 2018, flies started to appear in my living room during the evening. I have never had anything happen like this before. I have a very very clean house. The home is semi detached. It is now February 3, 2019. I have cleaned my house from top to bottom and sealed any spaces that might be vulnerable. There is not a space that I don't know about. However, the flies still get inside every evening. This is driving me insane. I am at the end of my tether and cannot solve this problem. I have scoured the internet for possible solutions but I have gotten nowhere. What else can I do? Can anyone give me any ideas I have not thought of already?
If they are cluster flies, you have probably sealed them up inside the home. They got in during the summer and laid eggs that are hatching now. Here is a product to get rid of cluster flies. You can probably locate something similar in the UK.