How to locate the cause of sewage smell?

[louisemh]

My living room smells like a sewer but I can't tell where it is coming from. Not sure where to start looking. The saniflo discharge pipe runs above but there is no sign of leakage on the ceiling. The main drain run from the manhole at the back runs under the kitchen, which has a solid floor, then under the living room and out to the front. It will be at least 1.5m below the floor of the living room as the pavement at the front is 2m below internal FFL. We have exposed floor boards in the living room but not sure how easy they will be to take up, plus I don't know which part of the floor void to investigate first. I could take the manhole cover off if that would be useful but its under some big paving slabs (on pedestals) so not the easiest thing. Could one of you lovely people give me some pointers. I don't want to pay for a CCTV survey straight away if there is something I can do myself first. Many thanks!!!!