We have recently moved into a new house and have since noticed that there is a sewage like smell coming from under the bottom step. This smell is not constant it comes and goes and is more apparent when the central heating is. Originally we thought it was a dead animal of sorts but since cutting holes in the bottom step there was nothing there apart from the floor boards. The bottom step appears to have some vents in them which were covered by the carpet and the air brick for the floor has been covered up at the front of the house. We poured a strong bleach down the drain to see if there was a blockage but there was not a smell in the house of the bleach.
Any help would be appreciated