How can I clean my overflow???

Double sink in my kitchen has been fine until recently, (nothing has been changed or added) but suddenly there is the most disgusting smell coming from the overflow pipe. I have removed the u-bend and cleaned a load of gunk from it, the pipe from the overflow has also been removed and unblocked however the smell is now a million times worse coming out of the overflow holes. "Drain Magic" was put down it to no avail. I have tried putting thick bleach down it, lots of boiling water and still nothing has worked. I thought about taking a small toothbrush to it, but the space between the sinks (from underneath) is so limited i don't think it would fit.

Could someone PLEASE give me some suggestions because I don't know how much longer I can cope with this vomit-inducing smell in my kitchen!!!