L am in a lst floor flat and have a sewage smell coming from the kitchen sink plugholes and from the cupboard underneath. Have put bio-enzyme cleaner down but only temporary cure. Smell not there every day, about 4 or 5 times a month.
L live very close to a tidal river and sometimes the smell is outside the apartments as well. Any ideas what this could be and how to remedy situation?

Is there a disposal installed? If there any noise from the trap when you drain? Let me know, Tom