Smelly drains bowl and a half sink

[Sisyphus]

My kitchen was built 35 years ago and has functioned well since then. However, for the last month or so the drains have started to smell and I cannot work out why.
I attach a photo of the setup. I can see that the two sinks go into a crossbar. The washing machine also goes into this crossbar and the only trap in the sytem is below the crossbar. I can remember the plumber saying that all this was necessary to get ensure there was no smell but I don't understand why it is suddenly not working. I have tried pouring cleaner into the sink but it makes no difference. I wonder if there is no water in the U trap but as there is very llittle room to unscrew he pipes, I am relucrtant to do anything until I understand the problem.
Can anyone offer me some advice as to the likely problem and how I can sort it.? I should also like to know what the "turret" sticking out of the pipe below the trap is for. There doesn't seem to be any access into the pipe but I assume that it must do something.
Many thanks.

 

[rpm]

Follow pipe from the main sink down through the shelf, just below the shelf to the right is a screw on blanking cap, there shouldn`t be a great deal of water but have a towel ready to catch any drips when you remove the cap.
I suspect the U-bend under this may be full of smelly gunk.

 

[gpbeck]

Don't think that trap system is 35 years old. And I doubt it was fitted by a proficient plumber. That 'horizontal' pipe just below the shelf is not level so there will be stale water at the right hand end. Really it should be above the shelf and horizontal with the u bend below it as in this illustration. The verticals to the basins can be shortened (and the horizontal) to fit it level. I'd swap the whole thing and tidy it up. In your case as fitted the U bend cannot be easily removed to clean it because it is too low and sitting on the cabinet base.