We have a newish downstairs loo, crappy extractor in the ceiling, and no other ventilation, although we tend to leave the door open. Noticed it getting musty in there about a month ago and now we have little flies which the Internet tells me are drain flies.
The sink in there isn't used much but the loo is regularly. Next door, in the utility, is a washing machine draining into the same waste pipe via an open topped vertical stand pipe.
Sort of hoping there's something I can too down the drains and loo that will kill them all dead - has anyone else dealt with a similar problem?