I have similar probs from time to time...
The outlet pipe of the outdoor inspection pit becomes partially blocked - the water level in the pit increases to about 2 feet (should be completely empty) and when we flush a toilet the water level in the WC pan rises alarmingly before very slowly draining away. This back pressure in the drain system is very porbably forcing some foul water back into your bath.
If you remove the cover of the inspection pit you will easily see if this is your problem. If it is wear old clothes and rubber gloves. Get hold of a good length of plastic water pipe (barrier pipe) and poke away at the blockage - I use a piece about 2 metres long and give it a good hose down afterwards. Takes me about 10 mins now but expect to spend up to an hour fishing around in the murky water if you're a first timer.
Good luck...
Peccavi
February 2009
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