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For time being use some Raid, fly zappers and fly paper. No harm calling a pest controller out also to look around.
Honey
Our first thought was rats, but it has been going on for years and years and we have no sign of rats hooray.
The walls are more than two feet thick, and the "room" is on a lower level than the rest of it.
Could the bluebottles be carried in a stream or something? We have got a disused well in the garden so maybe it goes underneath?
Green Man
If you're brave enough, take out a few bricks to better access through the access point. Then you can see if are any animal remains.
Maybe install a small cellar window just for air after you done everything.
Green Man
JK2006 wrote: Dead rats Honey.
I was thinking that JK, you're more than a pretty face.
Have a sort out in the celler and clear up the best you can. If your you and husband are good at repairing brick work, then do it. Seal any cracks, go around the celler with a pencil and slide it in gaps if goes through any holes then a rodent can come through it these are called entry points.
Don't store any food there in a celler like pet food. There has to be a reason for flies in the celler in which would be a food source.
I had this problem myself.
Also lay rat traps you want the ones that kills them, if you trap them and release they will come back.