How can I fix my sink???

I moved house a few months ago and have always found a smell in the kitchen, over the past couple of days it has revealed itself as being the sink. I have used drain cleaners, bleach, boiling water and eventually took the waste pipes apart. Eurgh!!!! They were totally choked with grease and stinky gunk! I also cleaned out all the other pipework before connecting it all together again. All the pipes would appear to be a the correct angle to allow proper drainage and the u-bend does have water in it at all times. Again, some more boiling water, just to make sure I had it all but guess what? THE SMELL IS STILL THERE!!!!!!!!!!!!!!!
Please, please please! Is there anyone out there with a suggestion that I haven't already tried (short of a new sink)???????

All suggestions greatly appreciated!