Help! Infestation of flies

[FelicityFeline]

For the last few days my downstairs loo has been infested with flies. Yesterday there were about 30, today there have been about double that already! There is no obvious source, but about a month ago there was a weird smell that came and went, and I wondered if something was dead under the floorboards. The smell eventually went but then the flies appeared.
I think they are either house flies, cluster flies or else small bluebottles. They are relatively slow moving and easy to swat.

Any tips on getting rid of them please? Happy to cordon it off for a few days and use the strong stuff, but I have three small kids so cannot let them near. Thank you!

The best thing I have found is sticky fly papers. No nasty fumes with these.

I'd spray with fly spray and shut the door. I think you correct that there is something dead in there! You need to check, find the source of the smell/flies and see how it got in there.

Thanks! It appears they are bluebottles, which seems to indicate even more that there is something dead. Anyhow, got a spray and sticky stuff so hopefully they will die off soon. Thank you