Unexplained Bathroom Issue

[Tigercubrider]

What I do to check for leaks is to pull off 2/4 sheets of toilet paper and fold and roll it along it's length, twisting. You end up with a short bit of dry paper rope. I tie this around a suspect pipe.
Even if a leak is slight, the paper will absorb the water and feel wet after a while

It is cheap and easy to check but probably open the windows while you have a bath/shower

Re decking: if you know where the manhole cover is, a plunge saw could cut a hatch area that you could refit with a couple of extra braces - if not, lidl/aldi have an inspection camera going cheap at the moment

 

[allthegearandnoidea]

Ok after a bit more investigation there is definitely a leak from the WC pan connector. I still don't understand how this would cause the foul smell when I run the sink tap, possibly something else is causing this issue.

As a first step I'll fix the wc pan connector leak, and probably pull up some floorboards to see how bad the damage is.

Many thanks.