We purchased a new house approx one year ago (it was newly built). Shortly after moving in (literally a couple of days) we began to get a bad smell in the downstairs bathroom which consists of a toilet and sink. I located the smell to the sink. The plumbers came, took off the sink and sealed around it again which took away the smell. However, it has now came back again. It isn't coming from the sink again, rather from the area where the sink pipe goes through the hole in the floor below it. The hole is well sealed with silicone but it still seems to be coming through. Also, our living room is adjacent to the toilet, and I can smell the same smell coming from the skirting boards in the living room, at the same point as the sink next door - if you know what I mean. The house is a timber framed house and is only one year old this December. I would appreciate if anyone coule give me any advice or information as to what this problem could be? Thank you. I am from ireland and we are coming into winter here which means it is getting a bit colder, if this has anything to do with it.

"we are coming into winter here which means it is getting a bit colder, if this has anything to do with it."
It just means that with cold weather the smell will seem worse since smells get heavier in cold weather and stays closer to the ground.
As for the smell inside your walls you will have to open up the wall and track down the leak. In the US we give a unconditional years warranty with our new homes. Contact your builder and see what he can do. Good luck, Tom