It sounds like it needs a good old airing underneath there too Bev. Can you move it at all? If so, then let that patch dry out, maybe chuck some diluted bleach on it and then hose it all through, go and get yourself a bag of pea beach gravel from B & Q or somewhere, just chuck that on the top and it should do the job for you! Just make sure you hose it through slowly so that it sinks in, and let it dry thoroughly before chucking the gravel on it. Good luck, another blooming nightmare for you then!

The soil may have been flattened and compacted by the artificial turf. I wouldn't have put it straight on to soil - it should have been laid on something like builders sand (thicker the layer the better for good drainage:

http://www.asgoodasgrass.co.uk/artificial-grass.php

....not that expensive and not that hard to do!

Just bumping this to see if anyone found a solution to the problem?

I'm not sure if it will help and I dont have grass in my garden,only slabs and block paving but once a week a make up a solution of biological soap powder or liqiud with hot water and soak all the slabs and paving for about an hour then rinse it thoroughly and it does destroy any smells as it breaks down the enzymes that cause the smell. You may have to improve the drainage by adding some sharp sand to the soil. Hope you manage to sort it before any warm weather arrives or your neighbors are not going to very pleased.

Check what type of soil you have. It may be clay based and no matter what you do it will be difficult to get it to drain away successfully. Years ago we had a sunken dog loo at mums & got a build up because it couldnt drain away properly :/

What we used to do when we lived at mums & her patio area got pongy, was last thing at night dilute a good bleach mix such as domestos in a large watering can with dash of washing
*up liquid. Wash the area down with this mix, leave overnight & in morning before dogs go out wash it down with clean water. It is great for killing weeds that may grow between paving stones as well.*

I use dilute Jeyes Fluid. It`ll kill any plants though.

I hate the smell of Jeyes fluid however it does work to remove the smells. I think your problem is simple drainage.
Try lifting the artifical grass and letting the air get too it.

When I get enough money together I am going to dig out the soil underneath and put in some gravel and then put the fake turf on top of that. I have been quoted approx £100 for the gravel though so its not cheap. Hopefully that will stop the smell problem though.
I have to be careful what I use to kill the smells as this area is up against my fence and the other side of my fence is my lawn and where the chemicals have drained under the fence it has killed my lawn.

Bad smells in garden? Try diluted fabric softener.

I keep hens and sometimes part of their run where its on concrete gets a bit whiffy so i just dilute fabric softener with hot water in a watering can and sprinkle on.

Madmare, have you tried diluting plenty of biological laundry powder in boiled water and then soaking it in that?

It might just do the trick. I'd try it every night for a bit then once a week. Failing that, bleach.