Bad Smell in new extension

chris1012

10 months back I extended my old kitchen/diner area to create a bigger open plan space and add a downstairs shower room with toilet.

Existing gully's had to be made redundant, complete new pipes for the plumbing and waste were installed and a new man hole was added into the garden so we didn't have to access the shared neighbors one (which they had built over).

Unfortunately a 'friend' did the job for me who I thought at the time I could trust and would not let me down, however 10months on and the problem I am about to tell you about has still not been resolved.

Since the works were complete, within the first couple of weeks of moving in we smelt a eggy smell in the kitchen dining area coming from behind the washing machine and boiler.

We thought it might of been a one off but basically since them 2 weeks, nearly every few days we have had smells on and off coming from behind the washing machine, under the boiler and bathroom.

The plumber after multiple visits (sub contracted from my 'friend') were unable to resolve the issue.
Initially they thought it was a trap issue and added traps to the boiler and washing machine (prior to the trap for the washing machine, they simply had a downward black pipe that the waste pipe of the washing machine fell loosely into).

Anyhow the friend was difficult to get a hold of and kept making empty/let down promises and a long story short, after 8 months of pushing they finally agreed to come down and remove two tiles near the washing machine to see if the old gulley was the issue (this was 5 weeks back).

Since then the smell is definitely noticeable from the removed tiles area, which is either near or where the old gulley was located.

I have had cctv drain surveys to look at my drains (at my own expese)but they were unable to find something that would indicate what the issue may be apart from a possible old redundant drain that looked like it had an orange seal at the start of it with plastic in front of it (the image didn't indicate if it was an issue but could potentially be).

There is also now a case of rats/mice which could potentially be entering from the cellar or the drains (were not sure as the cctv report didn't indicate any direct entry points).
The property had no previous history of these rodents and they are only heard and seems isolated to the new extension areas (please see pic).

I have been in touch with the owner of the property who lived in the house prior to selling and according to him, their was an eggy smell from the old kitchen area (exit point from behind the washing machine).
However as we have had completely new pipes installed, we believed this would not be the cause of the issue.

The building works were done with planning approval and building regs and all certs were supplied on completion.

Ideally I do not want to get the council involved in getting this issue resolved due to the history of friendship and making this more complicated.

Any advise of what we could do to rectify this?
We have spent an excess of 70k for all the works, furnishings, flooring etc and we can't live in this situation any more as we have a little baby and these smells are really getting to us!