How to investigate bad drain type smell

Hi, do you have a wash hand basin in the loo also? If so the waste from that may be connected into the toilet soil pipe meaning that when you flush the toilet it can cause the trap in the basin waste to be syphoned out leaving the bad smell into the room. if not that, there may be a faulty seal between the toilet and the soil. my advice would be to contact a plumber first to rule out any faults with the plumbing in the room. cheers josh Hewitts Heating.

Your wash hand basin trap and waste pipes could be the culprit, It could be getting siphoned out which can be fixed by fitting an antivac trap ,or the trap might be just dirty inside and need cleaning or renewing. Cheers Peter PPS plumbing

Your hand basin is more than likly connected the the wc waste so when you flush the wc the seal in the trap in the basin is being pulled and will make a glug glug noise there for your getting the smell coming ftom the soil stack

In my opinion the issue with flies is probably a fault with the drainage I would contact a drainage company to see what a survey would cost to see-if there is an issue can’t see there being a issue with the basin waste causing a smell as this would have accrued when installed hopefully this will be be helpful?