Stale smell in en suite bathroom

[headache56]

Over the past few months, I have been getting a stale smell, like wet-musty towels, in the bathroom. I have used bleach and caustic soda in all obvious orifices and the smell goes away for a week or so but then it comes back.

The smell seems to be coming from the plug hole in the hand basin so I removed the S-bend attached to the plug hole. There was some black damp / mould inside the fitting but this was only a very small amount.

I attach a photo of the pipes on the adjacent outside wall - I guess these are the pipes for the toilet outflow / stack and the shower tray waste and the hand basin waste-->
http://s630.photobucket.com/albums/uu28/headache5678_photos/?action=view&current=IMG_2392.jpg

(1)

I read comments on this site about flaps ? Could there be faulty "flaps" somewhere causing this smell if the sewage smell could maybe be coming up the shared stack ?

(2)
Is it possible to undo the waste (plug hole) in the shower tray once a shower tray is installed ?

 

[nogoodatfaultfinding]

if you have used the usual cleansers then it is probably static sat water which is not draining away.
Most likely causewould be no fall on the waste pipe from the basin.

 

[seco services]

(1)

I read comments on this site about flaps ? Could there be faulty "flaps" somewhere causing this smell if the sewage smell could maybe be coming up the shared stack ?

their won't be no flaps on your install.

(2)
Is it possible to undo the waste (plug hole) in the shower tray once a shower tray is installed ?

depends what type of waste you have.

have you checked all the traps have water in.
or the overflow part of the basin can hold muck and cause smellls.