Hi Bigyin Have you got a separate soak away?

If so has it been overflowing due to the recent downpoars!!

We had a similar problem recently and had to get a friend in to re dig our soakaway so that it did indeed drain away.

If it doesn't stagnant water sitting on the land will attract drain flies, you can always tell by the smell from the grey drain water, its worse that any toilet smell!!

See Link http://www.pestproducts.com/fliesdrains.htm