yes the toilet is downstairs at the back of the property. and the overflow pipe literally exits the top side of the toilet cistern and straight out the wall to outside. I did wonder if putting a tundish on it and putting it into the pipe from the cistern to the bowl might help the situation..
toilet is mains fed which runs through the house before reaching the toilet
If you think that the overflow is partly the problem you could replace the current flush mechanism with a more modern one which overflows down the pipe to the pan as an integral feature. This is likely to make no difference anyway.
Extractor fan is the key to any chance of success.