Suddenly the ensuite bathroom in our house has started to smell like sewage. Neither the toilet or shower is blocked, so wondering where the smell could be coming from? Is there any way the toilet could allow smells from the septic into the bathroom, like some one way valve has failed?

There "should" be a vent to the outside at the top of the soil pipe, ours is just an open pipe in the wall.

If the local wildlife has made its home in there it could stop the gases from the tank venting out and instead coming into the room.