Smell from inside vanity unit

[johnny2007]

Quick one, i have a very old vanity unit and a pedestal trap like this one

https://mcalpineplumbing.com/traps/tubular-traps/x10-straight-through-trap

In the past couple of days we noticed sewage smell coming from there.
All main sewage pipes are clear, no blockage anywhere.
There's no leak anywhere, everything is bone dry.
Tried running hot and cold water for 10 minutes, still bone dry.
I removed the trap and it was clean, despite having been there at least 16 years when we moved in.
Refitted it and same.
So my next action was to replace the blanking washer on the rodding eye with one from another trap.
The original washer has a slit all along the rim, that's how it's manufactured, it's not cut, see picture.
This washer, although not brittle, has become quite hard.
The one I fitted now hasn't got that slit, it's just a rubber disc.
Could this be the culprit or is there something else you can suggest?
I suppose if the problem was with the rodding eye washer I will know in the next few hours, but your input is appreciated.

 

[johnny2007]

Smell still present.
Any idea apart from removing the all pipework and redo?

 

[HERTS P&D]

Does the basin have an overflow? if so squirt some bleach into it.

Andy

 

[johnny2007]

It's an integrated overflow.
But the smell comes from inside the vanity unit, where the trap and pipework is.

 

[lostinthelight]

Got a picture from further out with pipes in place?

 

[johnny2007]

I'm going to remove the vanity unit and take a picture, bear with me please.
Thanks

 

[johnny2007]

Ok, here are the pictures.
After dismantling the vanity unit i found that the smell doesn't come from its pipework, but definitely from the area where the shower trap is.
No leak, all perfectly dry.
Any idea?

 

[johnny2007]

Pictures

 

[johnny2007]

Ok, further investigation.
I blocked the pipe from outside, from the rodding eye and using a party baloon.
Filled the shower tray with water and kept window open.
Then I shut the window and after a while the smell is still there, coming from the shower trap area.
I found that now it seeps from the joint with the shower tray, but I don't understand how the smell is still coming through with the pipe blocked and full of water.
Please give me some ideas.

 

[johnny2007]

Pictures