I was cleaning the bathroom and poured bleach down the toilet. I left it for about half and hour and when I flushed the toilet all the water drained away from the bowl and it made a gurgling noise - now the bathroom has got this dreadful gaseous smell!

The toilet is working now as it should but its such a dreadful smell

all the water drained away from the bowl and it made a gurgling noise
You have just described a blocked vent. Sounds like suction lowered the trap level and now you have sewer gas in your house. The solution would be to snake the lavatory roof vent. Put out enough snake to reach the base and 16 feet more. Good luck, n Tom