Hi & elcome to the forum. Have a look at the following links. I had problems with cluster flies which come around March and again in September. They cluster around Septmeber to October in your roof space. Cluster flies are slower than house flies and usualy are dead when you arrive at the van. one thing you must not do is close off any vents. Our flies were coming the the velux roof windows and through the roof vents.
Look for a fumer on AMazon, set off a couple in the van early evening.
Cluster flies are a cyclic thing, which you have to break. The more flies you have the more you will attract. You have to kill them off and the fumers will sort that out.
We were warned by our site that when we reopen in March we may be troubled by flies as it is apparently a common problem there. I don't know exactly where they are supposed to have set up for the Winter, but I shall go prepared to do battle!
__________________ Owner of country retreat/static caravan in the Lakes.
Look for a fumer on AMazon, set off a couple in the van early evening.
Cluster flies are a cyclic thing, which you have to break. The more flies you have the more you will attract. You have to kill them off and the fumers will sort that out.
They are likely to come back though.
Will look for a fumer and try and hope it works as will try anything to get rid of the critters.