Toilet overflow joining main waste?

[rhino666]

Hi

I am renting out a property and there has been a problem with a toilet.
New plastic waste fittings have been fitted to the toilet which is only a matter of 6-7 feet from the sewer it flows into.

The plumber has cut a hole into the waste pipe that leaves the toilet and inserted the cistern overflow pipe. This makes no sense to me as it appears that the noxious gases from the sewer would pass up the sewer pipe and through the overflow system into the house.

Can someone with more knowledge than me put me right on this one please.