To continue from my lovely previous thread...
www.mumsnet.com/Talk/property/4222687-Why-does-my-utility-room-smell-like-wee - my lovely new house smells of sewage in general, not just wee!
After having pest control out etc, this does not seem to be a major issue, however, it transpires the previous owners of this house built the extension over a manhole, which was capped off (maybe), covered with a board, but probably NOT properly covered in cement by their builders (and hence smells). Apparently they never had an issue with smells, & even used the affected room as a bedroom (!) but it smells like a sewer to me 
(not envy). If its this bad in April I think I need to get it sorted straight away or I'm in for a v unpleasant summer...
This will involve taking up a few slate tiles to get into the floor, digging into the space (if there is anything covering it) and then getting a new cover for the 'old' manhole and cementing over, then putting it all back
Has anyone ever had to do similar? What kind of trade would I even call to do this? And how can I make sure it is done right this time? Do I need a CCTV drain survey if I know where the manhole is, or is that just a waste of money? ( A new manhole was installed at time of extension, a few feet away in garden. That all seems to be fine & everything in house drains via that.
Also, if it is a shared manhole/sewer for others in the terrace, does it make a difference if it is 1st/last house (of 4), and how would I know what it is?
I'm also v afraid that once I open up the space rats will come out, so I think I need to do it then close it all up quickly... the owners suggested not retiling immediately to make sure that my fix solves the problem.
Thanks in advance for any suggestions