I remember years ago as a student a class mate (a mature student with children) had just moved into his brand new house around Easter. Just after the Easter holidays my class mate told us that there was a terrible smell coming from the utility room. For three or four weeks plumbers, builders and all sorts took the drainage system apart, opened manholes and washed out the drains. Nobody had any idea where the smell was coming from.

 

About a few days after his children returned to school his six-year-old son asked him if he could take the (real) egg he had decorated for Easter to school. The egg was proudly displayed in - yes you guessed it - the window cill in the utility room. The egg hadn’t been cleaned out properly and every time the sun shone through the utility room window a terrible smell was emitted from the egg.

 

So - after all the plumbers and builders coming through the house looking at the drains it turns out that the smell was coming from a real egg that hadn’t been cleaned out propane the smell was only apparent when the sun shone through the window and heated the egg up.

A dead mouse under a floor board or kitchen unit etc can smell really bad for a long time

9 minutes ago, ProDave said:

smell really bad for a long time

Mouse 2 days. Rat 7 days.

 

5 minutes ago, saveasteading said:

....  Rat 7 days.

 

Dependant on cannibalism I'm afraid. 

 

Rats will look at a dead stable mate and - in my experience within 24 hours - eat their dead Bessy Friend. Mostly I gather and dispose of all the rats shot in one session.

But I sometimes leave one or two rats I have shot as bait for others (saves my time). Rats are amazingly clean.

UPDATE: 

Builder was up and removed the 2 toilets with the plumber, he did say the original fittings were not great. Smell is no longer in bathrooms but still in the hotpress( where SVP is located) at random times throughout the day but it comes and goes. He removed the bottom of the plaster board the other day to expose the pipe but on that particular day there was no smell in hotpress. I came home today and went to hotpress and could smell the strong odour. Got on my hands and knees to smell bottom of pipe but couldn’t smell anything from the pipe itself however to the right of the SVP is the underfloor heating pipes and I could definitely smell the odour coming from the back of them. I believe the SVP goes under the floor I hotpress  and into the bathroom beside it. I mentioned this to builder but he can’t understand how the smell would come through so much concrete, insulation, radon barrier and floor screed . At this stage I believe a camera maybe be required.  I have attached picture of area 

Pester him to do that drain pressure test.  Only that will tell you if there is a leak in any part of the drainage system.  Without that he is poking around at random, getting nowhere and not solving the problem.

14 hours ago, ProDave said:

that drain pressure test. 

Builder seems to be avoiding that?